Summary:
The following metrics have been added:
vector_index_flush_write_bytes: the number of bytes that are written to chunk files during flushes.
vector_index_flush_us: the time that is taken to serialize and write one chunk file during a flush. Only the chunk write is covered, not the manifest update, since the manifest write is batched across chunks and cannot be attributed to a single flush.
vector_index_compact_us: the time that is taken by a compaction end to end, i.e. the merge, the merged-chunk write, the manifest update, the in-memory chunk swap, and the obsolete file cleanup being triggered.

The existing compact_read_bytes/compact_write_bytes increments were also moved down so they are fired together with compact_us after the compaction has succeeded.  This was done to avoid the counters being updated half way and on failure, they are still counted.

Test Plan: ./yb_build.sh release --cxx-test vector_lsm-test --gtest_filter '*SimpleCompactionMetrics*'

Reviewers: sergei

Reviewed By: sergei

Subscribers: svc_phabricator, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D55036