go/proxy: Add request/response RPC metrics

.changeset/chilled-mayflies-push.md

+---
+'@eth-optimism/proxyd': minor
+---
+
+Add request/response payload size metrics to proxyd

go/proxyd/metrics.go

@@ -2,10 +2,11 @@ package proxyd
 
 import (
 	"context"
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promauto"
 	"strconv"
 	"strings"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 )
 
 const (
@@ -20,6 +21,8 @@ const (
 	MethodUnknown = "unknown"
 )
 
+var PayloadSizeBuckets = []float64{10, 50, 100, 500, 1000, 5000, 10000, 100000, 1000000}
+
 var (
 	rpcRequestsTotal = promauto.NewCounter(prometheus.CounterOpts{
 		Namespace: MetricsNamespace,
@@ -139,6 +142,25 @@ var (
 		"source",
 	})
 
+	requestPayloadSizesGauge = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: MetricsNamespace,
+		Name:      "request_payload_sizes",
+		Help:      "Gauge of client request payload sizes.",
+		Buckets:   PayloadSizeBuckets,
+	}, []string{
+		"auth",
+		"method_name",
+	})
+
+	responsePayloadSizesGauge = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: MetricsNamespace,
+		Name:      "response_payload_sizes",
+		Help:      "Gauge of client response payload sizes.",
+		Buckets:   PayloadSizeBuckets,
+	}, []string{
+		"auth",
+	})
+
 	rpcSpecialErrors = []string{
 		"nonce too low",
 		"gas price too high",
@@ -185,3 +207,11 @@ func MaybeRecordSpecialRPCError(ctx context.Context, backendName, method string,
 		}
 	}
 }
+
+func RecordRequestPayloadSize(ctx context.Context, method string, payloadSize int) {
+	requestPayloadSizesGauge.WithLabelValues(GetAuthCtx(ctx), method).Observe(float64(payloadSize))
+}
+
+func RecordResponsePayloadSize(ctx context.Context, payloadSize int) {
+	responsePayloadSizesGauge.WithLabelValues(GetAuthCtx(ctx)).Observe(float64(payloadSize))
+}

go/proxyd/server.go

@@ -116,13 +116,15 @@ func (s *Server) HandleRPC(w http.ResponseWriter, r *http.Request) {
 		"user_agent", r.Header.Get("user-agent"),
 	)
 
-	req, err := ParseRPCReq(io.LimitReader(r.Body, s.maxBodySize))
+	bodyReader := &recordLenReader{Reader: io.LimitReader(r.Body, s.maxBodySize)}
+	req, err := ParseRPCReq(bodyReader)
 	if err != nil {
 		log.Info("rejected request with bad rpc request", "source", "rpc", "err", err)
 		RecordRPCError(ctx, BackendProxyd, MethodUnknown, err)
 		writeRPCError(ctx, w, nil, err)
 		return
 	}
+	RecordRequestPayloadSize(ctx, req.Method, bodyReader.Len)
 
 	group := s.rpcMethodMappings[req.Method]
 	if group == "" {
@@ -249,14 +251,17 @@ func writeRPCRes(ctx context.Context, w http.ResponseWriter, res *RPCRes) {
 	if res.IsError() && res.Error.HTTPErrorCode != 0 {
 		statusCode = res.Error.HTTPErrorCode
 	}
+
 	w.WriteHeader(statusCode)
-	enc := json.NewEncoder(w)
+	ww := &recordLenWriter{Writer: w}
+	enc := json.NewEncoder(ww)
 	if err := enc.Encode(res); err != nil {
 		log.Error("error writing rpc response", "err", err)
 		RecordRPCError(ctx, BackendProxyd, MethodUnknown, err)
 		return
 	}
 	httpResponseCodesTotal.WithLabelValues(strconv.Itoa(statusCode)).Inc()
+	RecordResponsePayloadSize(ctx, ww.Len)
 }
 
 func instrumentedHdlr(h http.Handler) http.HandlerFunc {
@@ -291,3 +296,25 @@ func GetXForwardedFor(ctx context.Context) string {
 	}
 	return xff
 }
+
+type recordLenReader struct {
+	io.Reader
+	Len int
+}
+
+func (r *recordLenReader) Read(p []byte) (n int, err error) {
+	n, err = r.Reader.Read(p)
+	r.Len += n
+	return
+}
+
+type recordLenWriter struct {
+	io.Writer
+	Len int
+}
+
+func (w *recordLenWriter) Write(p []byte) (n int, err error) {
+	n, err = w.Writer.Write(p)
+	w.Len += n
+	return
+}