Add OWNERS file to metrics constants proto

Adding yro@google.com as an OWNER to metrics constants proto to review
incoming events.

Test: Given ".." import, this change should not break anything.
Bug: 114138150

Change-Id: Ib4ddc153e102b03b7f78a83844023e80e1f9dc68
diff --git a/proto/src/metrics_constants/OWNERS b/proto/src/metrics_constants/OWNERS
new file mode 100644
index 0000000..9c70a5e
--- /dev/null
+++ b/proto/src/metrics_constants/OWNERS
@@ -0,0 +1,3 @@
+yanglu@google.com
+yaochen@google.com
+yro@google.com
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants/metrics_constants.proto
similarity index 100%
rename from proto/src/metrics_constants.proto
rename to proto/src/metrics_constants/metrics_constants.proto