Use the X-LaunchDarkly-User-Agent header for metrics if available (#65)

The javascript client sets this header.

relay.go

@@ -46,7 +46,8 @@ const (
 	defaultMetricsPrefix         = "launchdarkly_relay"
 	defaultFlushIntervalSecs     = 5
 
-	userAgentHeader = "user-agent"
+	userAgentHeader   = "user-agent"
+	ldUserAgentHeader = "X-LaunchDarkly-User-Agent"
 )
 
 var (
@@ -756,7 +757,7 @@ func bulkEventHandler(w http.ResponseWriter, req *http.Request) {
 func withGauge(handler http.HandlerFunc, measure metrics.Measure) http.HandlerFunc {
 	return func(w http.ResponseWriter, req *http.Request) {
 		ctx := getClientContext(req)
-		userAgent := req.Header.Get(userAgentHeader)
+		userAgent := getUserAgent(req)
 		metrics.WithGauge(ctx.getMetricsCtx(), userAgent, func() {
 			handler.ServeHTTP(w, req)
 		}, measure)
@@ -766,7 +767,7 @@ func withGauge(handler http.HandlerFunc, measure metrics.Measure) http.HandlerFu
 func withCount(handler http.HandlerFunc, measure metrics.Measure) http.HandlerFunc {
 	return func(w http.ResponseWriter, req *http.Request) {
 		ctx := getClientContext(req)
-		userAgent := req.Header.Get(userAgentHeader)
+		userAgent := getUserAgent(req)
 		metrics.WithCount(ctx.getMetricsCtx(), userAgent, func() {
 			handler.ServeHTTP(w, req)
 		}, measure)
@@ -789,7 +790,7 @@ func requestCountMiddleware(measure metrics.Measure) mux.MiddlewareFunc {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 			ctx := getClientContext(req)
-			userAgent := req.Header.Get(userAgentHeader)
+			userAgent := getUserAgent(req)
 			// Ignoring internal routing error that would have been ignored anyway
 			route, _ := mux.CurrentRoute(req).GetPathTemplate()
 			metrics.WithRouteCount(ctx.getMetricsCtx(), userAgent, route, req.Method, func() {
@@ -880,3 +881,11 @@ func obscureKey(key string) string {
 	}
 	return key
 }
+
+// getUserAgent returns the X-LaunchDarkly-User-Agent if available, falling back to the normal "User-Agent" header
+func getUserAgent(req *http.Request) string {
+	if agent := req.Header.Get(ldUserAgentHeader); agent != "" {
+		return agent
+	}
+	return req.Header.Get(userAgentHeader)
+}

relay_test.go

@@ -757,6 +757,20 @@ SdkKey = "sdk-98e2b0b4-2688-4a59-9810-1e0e3d798989"
 		"expected sdk key to be used as sdk key when both api key and sdk key are set")
 }
 
+func TestGetUserAgent(t *testing.T) {
+	t.Run("X-LaunchDarkly-User-Agent takes precedence", func(t *testing.T) {
+		req, _ := http.NewRequest("GET", "/", nil)
+		req.Header.Set(ldUserAgentHeader, "my-agent")
+		req.Header.Set(userAgentHeader, "something-else")
+		assert.Equal(t, "my-agent", getUserAgent(req))
+	})
+	t.Run("User-Agent is the fallback", func(t *testing.T) {
+		req, _ := http.NewRequest("GET", "/", nil)
+		req.Header.Set(userAgentHeader, "my-agent")
+		assert.Equal(t, "my-agent", getUserAgent(req))
+	})
+}
+
 // jsonFind returns the nested entity at a path in a json obj
 func jsonFind(obj map[string]interface{}, paths ...string) interface{} {
 	var value interface{} = obj