rpc: extract OpenTelemetry trace context from request headers (#33599)

This PR adds support for the extraction of OpenTelemetry trace context
from incoming JSON-RPC request headers, allowing geth spans to be linked
to upstream traces when present.

---------

Co-authored-by: lightclient <lightclient@protonmail.com>

rpc/http.go

@@ -30,6 +30,9 @@ import (
 	"strconv"
 	"sync"
 	"time"
+
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/propagation"
 )
 
 const (
@@ -334,6 +337,9 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 	ctx = context.WithValue(ctx, peerInfoContextKey{}, connInfo)
 
+	// Extract trace context from incoming headers.
+	ctx = otel.GetTextMapPropagator().Extract(ctx, propagation.HeaderCarrier(r.Header))
+
 	// All checks passed, create a codec that reads directly from the request body
 	// until EOF, writes the response to w, and orders the server to process a
 	// single request.

rpc/tracing_test.go

@@ -21,7 +21,9 @@ import (
 	"net/http/httptest"
 	"testing"
 
+	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/propagation"
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	"go.opentelemetry.io/otel/sdk/trace/tracetest"
 )
@@ -60,16 +62,33 @@ func newTracingServer(t *testing.T) (*Server, *sdktrace.TracerProvider, *tracete
 
 // TestTracingHTTP verifies that RPC spans are emitted when processing HTTP requests.
 func TestTracingHTTP(t *testing.T) {
-	t.Parallel()
+	// Not parallel: this test modifies the global otel TextMapPropagator.
+
+	// Set up a propagator to extract W3C Trace Context headers.
+	originalPropagator := otel.GetTextMapPropagator()
+	otel.SetTextMapPropagator(propagation.TraceContext{})
+	t.Cleanup(func() { otel.SetTextMapPropagator(originalPropagator) })
+
 	server, tracer, exporter := newTracingServer(t)
 	httpsrv := httptest.NewServer(server)
 	t.Cleanup(httpsrv.Close)
+
+	// Define the expected trace and span IDs for context propagation.
+	const (
+		traceID      = "4bf92f3577b34da6a3ce929d0e0e4736"
+		parentSpanID = "00f067aa0ba902b7"
+		traceparent  = "00-" + traceID + "-" + parentSpanID + "-01"
+	)
+
 	client, err := DialHTTP(httpsrv.URL)
 	if err != nil {
 		t.Fatalf("failed to dial: %v", err)
 	}
 	t.Cleanup(client.Close)
 
+	// Set trace context headers.
+	client.SetHeader("traceparent", traceparent)
+
 	// Make a successful RPC call.
 	var result echoResult
 	if err := client.Call(&result, "test_echo", "hello", 42, &echoArgs{S: "world"}); err != nil {
@@ -92,8 +111,10 @@ func TestTracingHTTP(t *testing.T) {
 		}
 	}
 	if rpcSpan == nil {
-		t.Fatalf("jsonrpc.test/echo span not found.")
+		t.Fatalf("jsonrpc.test/echo span not found")
 	}
+
+	// Verify span attributes.
 	attrs := attributeMap(rpcSpan.Attributes)
 	if attrs["rpc.system"] != "jsonrpc" {
 		t.Errorf("expected rpc.system=jsonrpc, got %v", attrs["rpc.system"])
@@ -107,6 +128,17 @@ func TestTracingHTTP(t *testing.T) {
 	if _, ok := attrs["rpc.jsonrpc.request_id"]; !ok {
 		t.Errorf("expected rpc.jsonrpc.request_id attribute to be set")
 	}
+
+	// Verify the span's parent matches the traceparent header values.
+	if got := rpcSpan.Parent.TraceID().String(); got != traceID {
+		t.Errorf("parent trace ID mismatch: got %s, want %s", got, traceID)
+	}
+	if got := rpcSpan.Parent.SpanID().String(); got != parentSpanID {
+		t.Errorf("parent span ID mismatch: got %s, want %s", got, parentSpanID)
+	}
+	if !rpcSpan.Parent.IsRemote() {
+		t.Error("expected parent span context to be marked as remote")
+	}
 }
 
 // TestTracingBatchHTTP verifies that RPC spans are emitted for batched JSON-RPC calls over HTTP.