Add replay block count metric (#7060)

beacon-chain/state/stategen/BUILD.bazel

@@ -30,6 +30,8 @@ go_library(
         "//shared/featureconfig:go_default_library",
         "//shared/params:go_default_library",
         "@com_github_pkg_errors//:go_default_library",
+        "@com_github_prometheus_client_golang//prometheus:go_default_library",
+        "@com_github_prometheus_client_golang//prometheus/promauto:go_default_library",
         "@com_github_prysmaticlabs_ethereumapis//eth/v1alpha1:go_default_library",
         "@com_github_sirupsen_logrus//:go_default_library",
         "@io_k8s_client_go//tools/cache:go_default_library",

beacon-chain/state/stategen/hot.go

@@ -4,6 +4,8 @@ import (
 	"context"
 
 	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 	ethpb "github.com/prysmaticlabs/ethereumapis/eth/v1alpha1"
 	"github.com/prysmaticlabs/prysm/beacon-chain/core/helpers"
 	"github.com/prysmaticlabs/prysm/beacon-chain/state"
@@ -13,6 +15,16 @@ import (
 	"go.opencensus.io/trace"
 )
 
+var (
+	replayBlockCount = promauto.NewHistogram(
+		prometheus.HistogramOpts{
+			Name:    "replay_blocks_count",
+			Help:    "The number of blocks to replay to generate a state",
+			Buckets: []float64{64, 256, 1024, 2048, 4096},
+		},
+	)
+)
+
 // HasState returns true if the state exists in cache or in DB.
 func (s *State) HasState(ctx context.Context, blockRoot [32]byte) (bool, error) {
 	if s.hotStateCache.Has(blockRoot) {
@@ -112,6 +124,8 @@ func (s *State) loadHotStateByRoot(ctx context.Context, blockRoot [32]byte) (*st
 		return nil, errors.Wrap(err, "could not load blocks for hot state using root")
 	}
 
+	replayBlockCount.Observe(float64(len(blks)))
+
 	return s.ReplayBlocks(ctx, startState, blks, targetSlot)
 }