add an auto-merge guard to the metrics constants file

Test: make
Bug: 63127271
Change-Id: Ib03a14d2a9e321d743a67f72f9be50259ced162a
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 4d9b0dd..9aa58f5 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -4190,6 +4190,8 @@
     // 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
   }