Remove obsolete comment and add Earliest and Latest timestamp.
Remove obsolete comment and add Earliest and Latest timestamp.

The latter is to eradicate the various MaxInt64 and MinInt64 in the
codebase.

asbjornenge/client_golang,telendt/client_golang,prometheus/client_golang,m3db/prometheus_client_golang,stapelberg/client_golang,mattkanwisher/client_golang,fabxc/client_golang,jimmidyson/prometheus-client_golang,KevinPike/client_golang