core/validatorapi: fix dynamic path proxy metric

core/validatorapi/metrics.go

@@ -64,9 +64,41 @@ func observeAPILatency(endpoint string) func() {
 func observeProxyAPILatency(path string) func() {
 	t0 := time.Now()
 
-	path = strings.Trim(strings.ReplaceAll(path, "/", "_"), "_")
+	label := proxyPathLabel(path)
 
 	return func() {
-		proxyAPILatency.WithLabelValues(path).Observe(time.Since(t0).Seconds())
+		proxyAPILatency.WithLabelValues(label).Observe(time.Since(t0).Seconds())
 	}
 }
+
+// proxyPathLabel converts a request path into a bounded metric label by replacing dynamic
+// path segments with placeholders. Without this, paths like /eth/v2/beacon/blocks/0x<root>
+// produce a unique label value per block root, which grows metric cardinality (and memory)
+// without bound.
+func proxyPathLabel(path string) string {
+	segments := strings.Split(strings.Trim(path, "/"), "/")
+	for i, segment := range segments {
+		switch {
+		case strings.HasPrefix(segment, "0x"):
+			segments[i] = "{hex}" // Block/state roots, validator pubkeys.
+		case isNumeric(segment):
+			segments[i] = "{n}" // Slots, epochs, validator indices.
+		case i > 0 && segments[i-1] == "peers":
+			segments[i] = "{peer_id}" // libp2p peer IDs are base58/base32, not hex or numeric.
+		default:
+		}
+	}
+
+	return strings.Join(segments, "_")
+}
+
+// isNumeric returns true if s is non-empty and contains only ASCII digits.
+func isNumeric(s string) bool {
+	if s == "" {
+		return false
+	}
+
+	return !strings.ContainsFunc(s, func(r rune) bool {
+		return r < '0' || r > '9'
+	})
+}

core/validatorapi/metrics_internal_test.go

@@ -0,0 +1,76 @@
+// Copyright © 2022-2026 Obol Labs Inc. Licensed under the terms of a Business Source License 1.1
+
+package validatorapi
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestProxyPathLabel(t *testing.T) {
+	tests := []struct {
+		name string
+		path string
+		want string
+	}{
+		{
+			name: "static path",
+			path: "/eth/v1/beacon/genesis",
+			want: "eth_v1_beacon_genesis",
+		},
+		{
+			name: "block by root collapses hex",
+			path: "/eth/v2/beacon/blocks/0x0342020caa311b9f104cd1b223872b7d416d868d2e5add744e7af8265ba435ff",
+			want: "eth_v2_beacon_blocks_{hex}",
+		},
+		{
+			name: "named block id kept",
+			path: "/eth/v2/beacon/blocks/head",
+			want: "eth_v2_beacon_blocks_head",
+		},
+		{
+			name: "numeric slot collapsed",
+			path: "/eth/v1/beacon/blocks/123456/root",
+			want: "eth_v1_beacon_blocks_{n}_root",
+		},
+		{
+			name: "pubkey collapses hex",
+			path: "/eth/v1/beacon/states/head/validators/0xa1b2c3",
+			want: "eth_v1_beacon_states_head_validators_{hex}",
+		},
+		{
+			name: "validator index collapsed",
+			path: "/eth/v1/validator/duties/attester/42",
+			want: "eth_v1_validator_duties_attester_{n}",
+		},
+		{
+			name: "peer id collapsed",
+			path: "/eth/v1/node/peers/QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N",
+			want: "eth_v1_node_peers_{peer_id}",
+		},
+		{
+			name: "peers list (no id) kept",
+			path: "/eth/v1/node/peers",
+			want: "eth_v1_node_peers",
+		},
+		{
+			name: "empty path",
+			path: "/",
+			want: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, proxyPathLabel(tt.path))
+		})
+	}
+}
+
+func TestProxyPathLabelBoundedCardinality(t *testing.T) {
+	// Distinct block roots must collapse to a single label value.
+	a := proxyPathLabel("/eth/v2/beacon/blocks/0x0342020caa311b9f104cd1b223872b7d416d868d2e5add744e7af8265ba435ff")
+	b := proxyPathLabel("/eth/v2/beacon/blocks/0x04639c0c1fff050014a818280fcd12dc8880077583e83fee738afd74ade618c0")
+	require.Equal(t, a, b)
+}