Adding stats for db pool
Adding stats for db pool

Summary: Adds DB Pool size stats to statsd

Test Plan: ran tests locally

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://phab.nylas.com/D1818

nylas/sync-engine,closeio/nylas,Eagles2F/sync-engine,nylas/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,gale320/sync-engine,closeio/nylas,jobscore/sync-engine,ErinCall/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,gale320/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,gale320/sync-engine,ErinCall/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,Eagles2F/sync-engine,closeio/nylas,closeio/nylas,nylas/sync-engine