Add timing metric for PublishBlockV2 endpoint (#15539)

This commit adds a Prometheus histogram metric to measure the processing
duration of the PublishBlockV2 beacon API endpoint in milliseconds.

The metric covers the complete request processing time including:
- Request validation and parsing
- Block decoding (SSZ/JSON)
- Broadcast validation checks
- Block proposal through ProposeBeaconBlock
- All synchronous operations and awaited goroutines

Background operations that run in goroutines (block broadcasting, blob
sidecar processing) are included in the timing since the main function
waits for their completion before returning.

Files changed:
- beacon-chain/rpc/eth/beacon/metrics.go: New metric definition
- beacon-chain/rpc/eth/beacon/handlers.go: Timing instrumentation
- beacon-chain/rpc/eth/beacon/BUILD.bazel: Added metrics.go and Prometheus deps
- changelog/potuz_add_publishv2_metric.md: Changelog entry

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

beacon-chain/rpc/eth/beacon/BUILD.bazel

@@ -8,6 +8,7 @@ go_library(
         "handlers_state.go",
         "handlers_validator.go",
         "log.go",
+        "metrics.go",
         "server.go",
     ],
     importpath = "github.com/OffchainLabs/prysm/v6/beacon-chain/rpc/eth/beacon",
@@ -61,6 +62,8 @@ go_library(
         "@com_github_ethereum_go_ethereum//common/hexutil:go_default_library",
         "@com_github_ethereum_go_ethereum//crypto/kzg4844: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_fastssz//:go_default_library",
         "@com_github_sirupsen_logrus//:go_default_library",
     ],

beacon-chain/rpc/eth/beacon/handlers.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/OffchainLabs/prysm/v6/api"
 	"github.com/OffchainLabs/prysm/v6/api/server/structs"
@@ -658,6 +659,12 @@ func (s *Server) PublishBlock(w http.ResponseWriter, r *http.Request) {
 // broadcast all given signed blobs. The broadcast behaviour may be adjusted via the
 // `broadcast_validation` query parameter.
 func (s *Server) PublishBlockV2(w http.ResponseWriter, r *http.Request) {
+	start := time.Now()
+	defer func() {
+		duration := time.Since(start).Milliseconds()
+		publishBlockV2Duration.Observe(float64(duration))
+	}()
+
 	ctx, span := trace.StartSpan(r.Context(), "beacon.PublishBlockV2")
 	defer span.End()
 	if shared.IsSyncing(r.Context(), w, s.SyncChecker, s.HeadFetcher, s.TimeFetcher, s.OptimisticModeFetcher) {

beacon-chain/rpc/eth/beacon/metrics.go

@@ -0,0 +1,16 @@
+package beacon
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	publishBlockV2Duration = promauto.NewHistogram(
+		prometheus.HistogramOpts{
+			Name:    "publish_block_v2_duration_milliseconds",
+			Help:    "Duration of publishBlockV2 endpoint processing in milliseconds",
+			Buckets: []float64{1, 5, 20, 100, 500, 1000, 2000, 5000},
+		},
+	)
+)

changelog/potuz_add_publishv2_metric.md

@@ -0,0 +1,3 @@
+### Added
+
+- Add timing metric `publish_block_v2_duration_milliseconds` to measure processing duration of the `PublishBlockV2` beacon API endpoint.