dht: add a prometheus metric for the size of the routing table

This is a bit hacky maybe we want to do this through DI ? Would be nice
to have this as a per DHT metric.

dht.go

@@ -14,8 +14,6 @@ import (
 	"github.com/libp2p/go-libp2p/core/peerstore"
 	"github.com/libp2p/go-libp2p/core/protocol"
 	"github.com/libp2p/go-libp2p/core/routing"
-	"go.opentelemetry.io/otel/attribute"
-	"go.opentelemetry.io/otel/trace"
 
 	"github.com/libp2p/go-libp2p-kad-dht/internal"
 	dhtcfg "github.com/libp2p/go-libp2p-kad-dht/internal/config"
@@ -39,6 +37,10 @@ import (
 	ma "github.com/multiformats/go-multiaddr"
 	"go.opencensus.io/tag"
 	"go.uber.org/zap"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/trace"
 )
 
 var (
@@ -153,6 +155,8 @@ type IpfsDHT struct {
 	// a bound channel to limit asynchronicity of in-flight ADD_PROVIDER RPCs
 	optProvJobsPool chan struct{}
 
+	rtSizeGauge prometheus.GaugeFunc
+
 	// configuration variables for tests
 	testAddressUpdateProcessing bool
 }
@@ -221,6 +225,17 @@ func New(ctx context.Context, h host.Host, options ...Option) (*IpfsDHT, error)
 		}
 	}
 
+	gauge := prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Namespace: "libp2p",
+		Subsystem: "dht_size",
+		Name:      "routing table size",
+		Help:      "The size of the full routing table.",
+	}, dht.getRtSizeForGauge)
+	if err := prometheus.Register(gauge); err == nil {
+		// Only unregister if register is successfull
+		dht.rtSizeGauge = gauge
+	}
+
 	// register for event bus and network notifications
 	sn, err := newSubscriberNotifiee(dht)
 	if err != nil {
@@ -372,6 +387,10 @@ func makeDHT(ctx context.Context, h host.Host, cfg dhtcfg.Config) (*IpfsDHT, err
 	return dht, nil
 }
 
+func (dht *IpfsDHT) getRtSizeForGauge() float64 {
+	return float64(dht.routingTable.Size())
+}
+
 // lookupCheck performs a lookup request to a remote peer.ID, verifying that it is able to
 // answer it correctly
 func (dht *IpfsDHT) lookupCheck(ctx context.Context, p peer.ID) error {
@@ -841,7 +860,11 @@ func (dht *IpfsDHT) RoutingTable() *kb.RoutingTable {
 
 // Close calls Process Close.
 func (dht *IpfsDHT) Close() error {
-	return dht.proc.Close()
+	err := dht.proc.Close()
+	if dht.rtSizeGauge != nil {
+		prometheus.Unregister(dht.rtSizeGauge)
+	}
+	return err
 }
 
 func mkDsKey(s string) ds.Key {

fullrt/dht.go

@@ -20,7 +20,7 @@ import (
 	"github.com/libp2p/go-libp2p/core/peerstore"
 	"github.com/libp2p/go-libp2p/core/protocol"
 	"github.com/libp2p/go-libp2p/core/routing"
-	swarm "github.com/libp2p/go-libp2p/p2p/net/swarm"
+	"github.com/libp2p/go-libp2p/p2p/net/swarm"
 
 	"github.com/gogo/protobuf/proto"
 	u "github.com/ipfs/boxo/util"
@@ -44,6 +44,8 @@ import (
 	"github.com/libp2p/go-libp2p-xor/kademlia"
 	kadkey "github.com/libp2p/go-libp2p-xor/key"
 	"github.com/libp2p/go-libp2p-xor/trie"
+
+	"github.com/prometheus/client_golang/prometheus"
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/codes"
 	"go.opentelemetry.io/otel/trace"
@@ -95,6 +97,8 @@ type FullRT struct {
 	bulkSendParallelism int
 
 	self peer.ID
+
+	rtSizeGauge prometheus.Gauge
 }
 
 // NewFullRT creates a DHT client that tracks the full network. It takes a protocol prefix for the given network,
@@ -197,6 +201,18 @@ func NewFullRT(h host.Host, protocolPrefix protocol.ID, options ...Option) (*Ful
 		self: self,
 	}
 
+	counter := prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "libp2p",
+		Subsystem: "dht_size",
+		Name:      "routing table size",
+		Help:      "The size of the full routing table.",
+	})
+	counter.Set(0)
+	if err := prometheus.Register(counter); err == nil {
+		// Only unregister if register is successfull
+		rt.rtSizeGauge = counter
+	}
+
 	rt.wg.Add(1)
 	go rt.runCrawler(ctx)
 
@@ -350,12 +366,19 @@ func (dht *FullRT) runCrawler(ctx context.Context) {
 		dht.rt = newRt
 		dht.lastCrawlTime = time.Now()
 		dht.rtLk.Unlock()
+
+		if dht.rtSizeGauge != nil {
+			dht.rtSizeGauge.Set(float64(len(m)))
+		}
 	}
 }
 
 func (dht *FullRT) Close() error {
 	dht.cancel()
 	err := dht.ProviderManager.Process().Close()
+	if dht.rtSizeGauge != nil {
+		prometheus.Unregister(dht.rtSizeGauge)
+	}
 	dht.wg.Wait()
 	return err
 }