Add missing `override` specifiers for `MetricsBase<T>::Add` overrides. am: f2f8f762eb am: 08cce9174d am: 401d118792

Original change: https://android-review.googlesource.com/c/platform/art/+/2458348

Change-Id: I7c1bcdb02f76c65d945c3be1ee097094c72c6b35
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>