[feature] Don't trace non-existing routes (#2172)

c.FullPath() is the empty string if a request doesn't match any route on
our mux. In those cases, there's no value in emitting a trace. The trace
will be empty, containing no other information beyond the fact that we
didn't match a route. Since Gin breaks off the processing early we don't
need to trace this request as it won't do anything and consumes no
further resources.

The 404 will still be emitted by our logs and will be visible from a
reverse proxy too.

internal/tracing/tracing.go

@@ -117,6 +117,15 @@ func InstrumentGin() gin.HandlerFunc {
 	)
 	propagator := otel.GetTextMapPropagator()
 	return func(c *gin.Context) {
+		spanName := c.FullPath()
+		// Do not trace a request if it didn't match a route. This doesn't omit
+		// all 404s as a request matching /:user for a user that doesn't exist
+		// still matches the route
+		if spanName == "" {
+			c.Next()
+			return
+		}
+
 		c.Set(tracerKey, tracer)
 		savedCtx := c.Request.Context()
 		defer func() {
@@ -127,13 +136,9 @@ func InstrumentGin() gin.HandlerFunc {
 			oteltrace.WithAttributes(httpconv.ServerRequest(config.GetHost(), c.Request)...),
 			oteltrace.WithSpanKind(oteltrace.SpanKindServer),
 		}
-		spanName := c.FullPath()
+
-		if spanName == "" {
-			spanName = fmt.Sprintf("HTTP %s route not found", c.Request.Method)
-		} else {
 		rAttr := semconv.HTTPRoute(spanName)
 		opts = append(opts, oteltrace.WithAttributes(rAttr))
-		}
 		id := gtscontext.RequestID(c.Request.Context())
 		if id != "" {
 			opts = append(opts, oteltrace.WithAttributes(attribute.String("requestID", id)))