Support caller-defined namespaces in merkledb (#3747)

Signed-off-by: Joshua Kim <20001595+joshua-kim@users.noreply.github.com>
Co-authored-by: Stephen Buttolph <stephen@avalabs.org>

Author: joshua-kim

x/merkledb/db.go

@@ -190,6 +190,7 @@ type Config struct {
 	// Prometheus.
 	// This may be useful for testing.
 	Reg        prometheus.Registerer
+	Namespace  string
 	TraceLevel TraceLevel
 	Tracer     trace.Tracer
 }
@@ -245,7 +246,7 @@ type merkleDB struct {
 
 // New returns a new merkle database.
 func New(ctx context.Context, db database.Database, config Config) (MerkleDB, error) {
-	metrics, err := newMetrics("merkledb", config.Reg)
+	metrics, err := newMetrics(config.Namespace, config.Reg)
 	if err != nil {
 		return nil, err
 	}

x/merkledb/metrics.go

@@ -8,6 +8,8 @@ import (
 	"sync"
 
 	"github.com/prometheus/client_golang/prometheus"
+
+	"github.com/ava-labs/avalanchego/utils/metric"
 )
 
 const (
@@ -93,11 +95,13 @@ type prometheusMetrics struct {
 	lookup *prometheus.CounterVec
 }
 
-func newMetrics(namespace string, reg prometheus.Registerer) (metrics, error) {
+func newMetrics(prefix string, reg prometheus.Registerer) (metrics, error) {
 	// TODO: Should we instead return an error if reg is nil?
 	if reg == nil {
 		return &mockMetrics{}, nil
 	}
+
+	namespace := metric.AppendNamespace(prefix, "merkledb")
 	m := prometheusMetrics{
 		hashes: prometheus.NewCounter(prometheus.CounterOpts{
 			Namespace: namespace,