Background Merges and Mutations No Longer Increment SelectedRows and SelectedBytes Metrics

Now, background merges, mutations and OPTIMIZE will not increment SelectedRows and SelectedBytes metrics. They (still) will increment MergedRows and MergedUncompressedBytes as it was before. This only affects the metric values, and makes them better. This change does not introduce any incompatibility, but you may wonder about the changes of metrics, so we put in this category. #37040 (Nikolai Kochetov).