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/
(cherry picked from commit 998f97f41d1d2c779202155e9c55361919eff365)
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 5319086..6167d85 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -4252,6 +4252,11 @@
     // 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
   }