Add overall metric for message sync latency
Add overall metric for message sync latency

In addition to per account metrics, it would be nice to have an aggregated metric 'overall'

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