func NopMetrics() *Metrics {
	return &Metrics{
		Size:         discard.NewGauge(),
		TxSizeBytes:  discard.NewHistogram(),
		FailedTxs:    discard.NewCounter(),
		RejectedTxs:  discard.NewCounter(),
		EvictedTxs:   discard.NewCounter(),
		RecheckTimes: discard.NewCounter(),
	}
}
NopMetrics returns no-op Metrics.
package mempool
import (
	"github.com/go-kit/kit/metrics"
	"github.com/go-kit/kit/metrics/discard"
	"github.com/go-kit/kit/metrics/prometheus"
	stdprometheus "github.com/prometheus/client_golang/prometheus"
)
const (
	MetricsSubsystem = "mempool"
)
type Metrics struct {
	Size metrics.Gauge
	TxSizeBytes metrics.Histogram
	FailedTxs metrics.Counter
	RejectedTxs metrics.Counter
	EvictedTxs metrics.Counter
	RecheckTimes metrics.Counter
}
func PrometheusMetrics(namespace string, labelsAndValues ...string) *Metrics {
	labels := []string{}
	for i := 0; i < len(labelsAndValues); i += 2 {
		labels = append(labels, labelsAndValues[i])
	}
	return &Metrics{
		Size: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
			Namespace: namespace,
			Subsystem: MetricsSubsystem,
			Name:      "size",
			Help:      "Size of the mempool (number of uncommitted transactions).",
		}, labels).With(labelsAndValues...),
		TxSizeBytes: prometheus.NewHistogramFrom(stdprometheus.HistogramOpts{
			Namespace: namespace,
			Subsystem: MetricsSubsystem,
			Name:      "tx_size_bytes",
			Help:      "Transaction sizes in bytes.",
			Buckets:   stdprometheus.ExponentialBuckets(1, 3, 17),
		}, labels).With(labelsAndValues...),
		FailedTxs: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
			Namespace: namespace,
			Subsystem: MetricsSubsystem,
			Name:      "failed_txs",
			Help:      "Number of failed transactions.",
		}, labels).With(labelsAndValues...),
		RejectedTxs: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
			Namespace: namespace,
			Subsystem: MetricsSubsystem,
			Name:      "rejected_txs",
			Help:      "Number of rejected transactions.",
		}, labels).With(labelsAndValues...),
		EvictedTxs: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
			Namespace: namespace,
			Subsystem: MetricsSubsystem,
			Name:      "evicted_txs",
			Help:      "Number of evicted transactions.",
		}, labels).With(labelsAndValues...),
		RecheckTimes: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
			Namespace: namespace,
			Subsystem: MetricsSubsystem,
			Name:      "recheck_times",
			Help:      "Number of times transactions are rechecked in the mempool.",
		}, labels).With(labelsAndValues...),
	}
}