Hotcold: Added ability to select which levels record metrics

Hotcold: Added ability to select which levels record metrics

Summary: Added an option to `Options` to specify which levels have metrics recorded and used that option in `Version::Get()` to stop metrics being recorded for wrong levels and in `DBImpl` to stop `DoCompactionWork()` trying to split on hotcold for levels where no metrics are available.

Test Plan: make check

Reviewers: vamsi, dhruba

Reviewed By: vamsi

CC: leveldb
Differential Revision: https://reviews.facebook.net/D8511