metrics: Update doc for timestamp format
metrics: Update doc for timestamp format

The parameters "start" and "end" also accept the format YYYYMMDDHH.
See https://phabricator.wikimedia.org/T177671

Pchelolo/restbase,cscott/restbase,wikimedia/mediawiki-services-restbase,wikimedia/restbase,wikimedia/restbase,milimetric/restbase,d00rman/restbase,milimetric/restbase,wikimedia/mediawiki-services-restbase,Pchelolo/restbase,cscott/restbase,d00rman/restbase