Add missing `override` specifiers for `MetricsBase<T>::Add` overrides.

(cherry picked from commit 9c922191ff7cae293e5e56bd42c8fcb778f01998)

Test: mmm art
Bug: 270391874
Change-Id: Ifef16a1548bd187aee34f1cf5d39bec0d8ffb50f
Merged-In: Ifef16a1548bd187aee34f1cf5d39bec0d8ffb50f
1 file changed