This removes text parsing in leveldb metrics collection code. All metrics
can now be accessed through the stats API provided by leveldb.

We also add new gauge-typed metrics that count the number of tables at each level.

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Co-authored-by: Exca-DK <Exca-DK@users.noreply.github.com>
Co-authored-by: Gary Rong <garyrong0905@gmail.com>
Co-authored-by: Felix Lange <fjl@twurst.com>