fix: Only set incoming user agent if not already present (#1366)

Following up to the following PR, we should only set the incoming user
agent if the key does not already exist. This is useful in certain
scenarios when two Refinery's are connected together.

- #1358

- Only set incoming user agent meta field if the event doesn't already
have a value for it
- Add unit test to verify behaviour
- Update existing tests to set custom user-agent and verify it's set
correctly

route/route.go

@@ -1036,7 +1036,7 @@ func getUserAgentFromRequest(req *http.Request) string {
 }
 
 func addIncomingUserAgent(ev *types.Event, userAgent string) {
-	if userAgent != "" {
+	if userAgent != "" && ev.Data["meta.refinery.incoming_user_agent"] == nil {
 		ev.Data["meta.refinery.incoming_user_agent"] = userAgent
 	}
 }

route/route_test.go

@@ -744,3 +744,25 @@ func TestAddIncomingUserAgent(t *testing.T) {
 		require.Equal(t, "test-agent", event.Data["meta.refinery.incoming_user_agent"])
 	})
 }
+
+func TestAddIncomingUserAgent(t *testing.T) {
+	t.Run("no incoming user agent", func(t *testing.T) {
+		event := &types.Event{
+			Data: map[string]interface{}{},
+		}
+
+		addIncomingUserAgent(event, "test-agent")
+		require.Equal(t, "test-agent", event.Data["meta.refinery.incoming_user_agent"])
+	})
+
+	t.Run("existing incoming user agent", func(t *testing.T) {
+		event := &types.Event{
+			Data: map[string]interface{}{
+				"meta.refinery.incoming_user_agent": "test-agent",
+			},
+		}
+
+		addIncomingUserAgent(event, "another-test-agent")
+		require.Equal(t, "test-agent", event.Data["meta.refinery.incoming_user_agent"])
+	})
+}