[metrics] Move existing command line arguments to flags

This will allow us to tune metrics reporting behavior as needed
through server-side configurations rather than having to wait for the
next ART release.

This CL also includes the flag to enable statsd reporting, even though
that feature is not yet implemented.

Bug: 175050458
Test: manual
Change-Id: Id69828684f2cf9d32471703494439d0722f18dce
8 files changed