Session pool metrics registered multiple times
The new metrics that have been added for the `SessionPool` are added multiple times if multiple different session pools are created by a `Spanner` instance. This could happen if the user creates database clients for different databases, or if multiple database clients using different `SessionPoolOptions` are requested. This again will lead to an `InvalidArgumentException` being thrown. See for example https://source.cloud.google.com/results/invocations/a8dc0694-336f-483b-bb8f-2ee502506a2b/targets/cloud-devrel%2Fjava%2Fjava-docs-samples%2Fjava8%2Fpresubmit/log
diff --git a/google-cloud-spanner/src/main/java/com/google/cloud/spanner/MetricRegistryConstants.java b/google-cloud-spanner/src/main/java/com/google/cloud/spanner/MetricRegistryConstants.java
index 60111618..5902598c 100644
--- a/google-cloud-spanner/src/main/java/com/google/cloud/spanner/MetricRegistryConstants.java
+++ b/google-cloud-spanner/src/main/java/com/google/cloud/spanner/MetricRegistryConstants.java
@@ -15,6 +15,7 @@
  */
 package com.google.cloud.spanner;
 
+import com.google.api.gax.core.GaxProperties;
 import com.google.common.collect.ImmutableList;
 import io.opencensus.metrics.LabelKey;
 import io.opencensus.metrics.LabelValue;
@@ -23,20 +24,13 @@ import io.opencensus.metrics.LabelValue;
 class MetricRegistryConstants {
 
   // The label keys are used to uniquely identify timeseries.
-  private static final LabelKey DATABASE = LabelKey.create("database", "Target database");
-  private static final LabelKey INSTANCE_ID =
-      LabelKey.create("instance_id", "Name of the instance");
   private static final LabelKey LIBRARY_VERSION =
       LabelKey.create("library_version", "Library version");
 
-  /** The label value is used to represent missing value. */
-  private static final LabelValue UNSET_LABEL = LabelValue.create(null);
-
-  static final ImmutableList<LabelKey> SPANNER_LABEL_KEYS =
-      ImmutableList.of(DATABASE, INSTANCE_ID, LIBRARY_VERSION);
+  static final ImmutableList<LabelKey> SPANNER_LABEL_KEYS = ImmutableList.of(LIBRARY_VERSION);
 
   static final ImmutableList<LabelValue> SPANNER_DEFAULT_LABEL_VALUES =
-      ImmutableList.of(UNSET_LABEL, UNSET_LABEL, UNSET_LABEL);
+      ImmutableList.of(LabelValue.create(GaxProperties.getLibraryVersion(SpannerImpl.class)));
 
   /** Unit to represent counts. */
   static final String COUNT = "1";
diff --git a/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java b/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java
index 5023f0fa..729b01f7 100644
--- a/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java
+++ b/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java
@@ -69,10 +69,13 @@ import io.opencensus.trace.Span;
 import io.opencensus.trace.Status;
 import io.opencensus.trace.Tracer;
 import io.opencensus.trace.Tracing;
+import java.util.Arrays;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Map;
 import java.util.Queue;
 import java.util.Random;
 import java.util.Set;
@@ -1088,6 +1091,11 @@ final class SessionPool {
     RANDOM;
   }
 
+  private static final Object POOLS_LOCK = new Object();
+
+  @GuardedBy("POOLS_LOCK")
+  private static final Map<MetricRegistry, List<SessionPool>> REGISTERED_POOLS = new HashMap<>();
+
   private final SessionPoolOptions options;
   private final SessionClient sessionClient;
   private final ScheduledExecutorService executor;
@@ -1150,15 +1158,12 @@ final class SessionPool {
    * Return pool is immediately ready for use, though getting a session might block for sessions to
    * be created.
    */
-  static SessionPool createPool(
-      SpannerOptions spannerOptions, SessionClient sessionClient, List<LabelValue> labelValues) {
+  static SessionPool createPool(SpannerOptions spannerOptions, SessionClient sessionClient) {
     return createPool(
         spannerOptions.getSessionPoolOptions(),
         ((GrpcTransportOptions) spannerOptions.getTransportOptions()).getExecutorFactory(),
         sessionClient,
-        new Clock(),
-        Metrics.getMetricRegistry(),
-        labelValues);
+        new Clock());
   }
 
   static SessionPool createPool(
@@ -1210,6 +1215,14 @@ final class SessionPool {
       Clock clock,
       MetricRegistry metricRegistry,
       List<LabelValue> labelValues) {
+    synchronized (POOLS_LOCK) {
+      if (!REGISTERED_POOLS.containsKey(metricRegistry)) {
+        initMetricsCollection(metricRegistry, labelValues);
+        REGISTERED_POOLS.put(metricRegistry, new LinkedList<>(Arrays.asList(this)));
+      } else {
+        REGISTERED_POOLS.get(metricRegistry).add(this);
+      }
+    }
     this.options = options;
     this.executorFactory = executorFactory;
     this.executor = executor;
@@ -1229,7 +1242,6 @@ final class SessionPool {
     this.sessionClient = sessionClient;
     this.clock = clock;
     this.poolMaintainer = new PoolMaintainer();
-    this.initMetricsCollection(metricRegistry, labelValues);
   }
 
   @VisibleForTesting
@@ -1862,11 +1874,36 @@ final class SessionPool {
     }
   }
 
+  private static final class Sum implements ToLongFunction<Void> {
+    private final MetricRegistry registry;
+    private final Function<SessionPool, Long> function;
+
+    static Sum of(MetricRegistry registry, Function<SessionPool, Long> function) {
+      return new Sum(registry, function);
+    }
+
+    private Sum(MetricRegistry registry, Function<SessionPool, Long> function) {
+      this.registry = registry;
+      this.function = function;
+    }
+
+    @Override
+    public long applyAsLong(Void input) {
+      long res = 0L;
+      synchronized (POOLS_LOCK) {
+        for (SessionPool pool : REGISTERED_POOLS.get(registry)) {
+          res += function.apply(pool);
+        }
+      }
+      return res;
+    }
+  };
+
   /**
    * Initializes and creates Spanner session relevant metrics. When coupled with an exporter, it
    * allows users to monitor client behavior.
    */
-  private void initMetricsCollection(MetricRegistry metricRegistry, List<LabelValue> labelValues) {
+  static void initMetricsCollection(MetricRegistry metricRegistry, List<LabelValue> labelValues) {
     DerivedLongGauge maxInUseSessionsMetric =
         metricRegistry.addDerivedLongGauge(
             MAX_IN_USE_SESSIONS,
@@ -1925,68 +1962,80 @@ final class SessionPool {
     // invoked whenever metrics are collected.
     maxInUseSessionsMetric.createTimeSeries(
         labelValues,
-        this,
-        new ToLongFunction<SessionPool>() {
-          @Override
-          public long applyAsLong(SessionPool sessionPool) {
-            return sessionPool.maxSessionsInUse;
-          }
-        });
+        null,
+        Sum.of(
+            metricRegistry,
+            new Function<SessionPool, Long>() {
+              @Override
+              public Long apply(SessionPool input) {
+                return Long.valueOf(input.maxSessionsInUse);
+              }
+            }));
 
     // The value of a maxSessions is observed from a callback function. This function is invoked
     // whenever metrics are collected.
     maxAllowedSessionsMetric.createTimeSeries(
         labelValues,
-        options,
-        new ToLongFunction<SessionPoolOptions>() {
-          @Override
-          public long applyAsLong(SessionPoolOptions options) {
-            return options.getMaxSessions();
-          }
-        });
+        null,
+        Sum.of(
+            metricRegistry,
+            new Function<SessionPool, Long>() {
+              @Override
+              public Long apply(SessionPool input) {
+                return Long.valueOf(input.options.getMaxSessions());
+              }
+            }));
 
     // The value of a numSessionsInUse is observed from a callback function. This function is
     // invoked whenever metrics are collected.
     numInUseSessionsMetric.createTimeSeries(
         labelValues,
-        this,
-        new ToLongFunction<SessionPool>() {
-          @Override
-          public long applyAsLong(SessionPool sessionPool) {
-            return sessionPool.numSessionsInUse;
-          }
-        });
+        null,
+        Sum.of(
+            metricRegistry,
+            new Function<SessionPool, Long>() {
+              @Override
+              public Long apply(SessionPool input) {
+                return Long.valueOf(input.numSessionsInUse);
+              }
+            }));
 
     // The value of a numWaiterTimeouts is observed from a callback function. This function is
     // invoked whenever metrics are collected.
     sessionsTimeouts.createTimeSeries(
         labelValues,
-        this,
-        new ToLongFunction<SessionPool>() {
-          @Override
-          public long applyAsLong(SessionPool sessionPool) {
-            return sessionPool.getNumWaiterTimeouts();
-          }
-        });
+        null,
+        Sum.of(
+            metricRegistry,
+            new Function<SessionPool, Long>() {
+              @Override
+              public Long apply(SessionPool input) {
+                return input.getNumWaiterTimeouts();
+              }
+            }));
 
     numAcquiredSessionsMetric.createTimeSeries(
         labelValues,
-        this,
-        new ToLongFunction<SessionPool>() {
-          @Override
-          public long applyAsLong(SessionPool sessionPool) {
-            return sessionPool.numSessionsAcquired;
-          }
-        });
+        null,
+        Sum.of(
+            metricRegistry,
+            new Function<SessionPool, Long>() {
+              @Override
+              public Long apply(SessionPool input) {
+                return input.numSessionsAcquired;
+              }
+            }));
 
     numReleasedSessionsMetric.createTimeSeries(
         labelValues,
-        this,
-        new ToLongFunction<SessionPool>() {
-          @Override
-          public long applyAsLong(SessionPool sessionPool) {
-            return sessionPool.numSessionsReleased;
-          }
-        });
+        null,
+        Sum.of(
+            metricRegistry,
+            new Function<SessionPool, Long>() {
+              @Override
+              public Long apply(SessionPool input) {
+                return input.numSessionsReleased;
+              }
+            }));
   }
 }
diff --git a/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java b/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java
index 08089c89..0178d26a 100644
--- a/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java
+++ b/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java
@@ -16,7 +16,6 @@
 
 package com.google.cloud.spanner;
 
-import com.google.api.gax.core.GaxProperties;
 import com.google.api.gax.paging.Page;
 import com.google.cloud.BaseService;
 import com.google.cloud.PageImpl;
@@ -28,11 +27,9 @@ import com.google.cloud.spanner.spi.v1.SpannerRpc.Paginated;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
-import com.google.common.collect.ImmutableList;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.ListenableFuture;
 import com.google.spanner.v1.ExecuteSqlRequest.QueryOptions;
-import io.opencensus.metrics.LabelValue;
 import io.opencensus.trace.Tracer;
 import io.opencensus.trace.Tracing;
 import java.util.ArrayList;
@@ -153,14 +150,8 @@ class SpannerImpl extends BaseService<SpannerOptions> implements Spanner {
       if (dbClients.containsKey(db)) {
         return dbClients.get(db);
       } else {
-        List<LabelValue> labelValues =
-            ImmutableList.of(
-                LabelValue.create(db.getDatabase()),
-                LabelValue.create(db.getInstanceId().getName()),
-                LabelValue.create(GaxProperties.getLibraryVersion(getOptions().getClass())));
         SessionPool pool =
-            SessionPool.createPool(
-                getOptions(), SpannerImpl.this.getSessionClient(db), labelValues);
+            SessionPool.createPool(getOptions(), SpannerImpl.this.getSessionClient(db));
         DatabaseClientImpl dbClient = createDatabaseClient(pool);
         dbClients.put(db, dbClient);
         return dbClient;
diff --git a/google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolTest.java b/google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolTest.java
index 2047b6c8..7a507d80 100644
--- a/google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolTest.java
+++ b/google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolTest.java
@@ -1577,11 +1577,7 @@ public class SessionPoolTest extends BaseSessionPoolTest {
     FakeClock clock = new FakeClock();
     clock.currentTimeMillis = System.currentTimeMillis();
     FakeMetricRegistry metricRegistry = new FakeMetricRegistry();
-    List<LabelValue> labelValues =
-        Arrays.asList(
-            LabelValue.create("database1"),
-            LabelValue.create("instance1"),
-            LabelValue.create("1.0.0"));
+    List<LabelValue> labelValues = Arrays.asList(LabelValue.create("1.0.0"));
 
     setupMockSessionCreation();
     pool = createPool(clock, metricRegistry, labelValues);
['google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolTest.java', 'google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java', 'google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java', 'google-cloud-spanner/src/main/java/com/google/cloud/spanner/MetricRegistryConstants.java']