Add metrics constant for data plan usage screen.

Change-Id: Ia5d13e4c3a8783e5c920791e7c2d6ef9b511b0bb
Note: This change is required for Data balance UI.
Bug: 62349208
Test: mmma frameworks/base/proto/

proto/src/metrics_constants.proto

@@ -4252,6 +4252,11 @@ message MetricsEvent {
     // OS: O MR
     FIELD_RANKED_POSITION = 1087;
 
+    // OPEN: Settings > Data plan usage
+    // CATEGORY: SETTINGS
+    // OS: O MR
+    DATA_PLAN_USAGE_SUMMARY = 1088;
+
     // Add new aosp constants above this line.
     // END OF AOSP CONSTANTS
   }