Micrometer JSON registry uses a non-blocking route
### Describe the bug

... so if there are gauges which require a blocking operation to get the value, then the call to `/metrics` fails

### Expected behavior

_No response_

### Actual behavior

_No response_

### How to Reproduce?

Use `quarkus.micrometer.export.json.enabled=true` and register a gauge that requires a blocking operation to retrieve the value, and then request a JSON export from `/metrics`

### Output of `uname -a` or `ver`

_No response_

### Output of `java -version`

_No response_

### GraalVM version (if different from Java)

_No response_

### Quarkus version or git rev

_No response_

### Build tool (ie. output of `mvnw --version` or `gradlew --version`)

_No response_

### Additional information

see discussion https://github.com/quarkusio/quarkus/discussions/23605
diff --git a/extensions/micrometer/deployment/src/main/java/io/quarkus/micrometer/deployment/export/JsonRegistryProcessor.java b/extensions/micrometer/deployment/src/main/java/io/quarkus/micrometer/deployment/export/JsonRegistryProcessor.java
index 6b0ce9ad374..d97cf11de29 100644
--- a/extensions/micrometer/deployment/src/main/java/io/quarkus/micrometer/deployment/export/JsonRegistryProcessor.java
+++ b/extensions/micrometer/deployment/src/main/java/io/quarkus/micrometer/deployment/export/JsonRegistryProcessor.java
@@ -45,6 +45,7 @@ public void initializeJsonRegistry(MicrometerConfig config,
         routes.produce(nonApplicationRootPathBuildItem.routeBuilder()
                 .routeFunction(config.export.json.path, recorder.route())
                 .handler(recorder.getHandler())
+                .blockingRoute()
                 .build());
 
         log.debug("Initialized a JSON meter registry on path="
['extensions/micrometer/deployment/src/main/java/io/quarkus/micrometer/deployment/export/JsonRegistryProcessor.java']