add an auto-merge guard to the metrics constants file

Test: make
Bug: 63127271
Change-Id: Ib03a14d2a9e321d743a67f72f9be50259ced162a

proto/src/metrics_constants.proto

@@ -4190,6 +4190,8 @@ message MetricsEvent {
     // OS: O DR
     FIELD_TIME_ELAPSED_BETWEEN_CHARGE_MS = 1030;
 
+    // ---- End O-DR1 Constants, all O-DR1 constants go above this line ----
+
     // Add new aosp constants above this line.
     // END OF AOSP CONSTANTS
   }