Add missing `override` specifiers for `MetricsBase<T>::Add` overrides. am: f2f8f762eb

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

Change-Id: I0fff22921d71efe0def3595faaf2b2275be6a563
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>