Report mkfile metrics with --mk-metrics

Combined with roboleaf CI changes to pass this flag and subsequent
pipeline changes, this will allow basic mkfile metrics to be
dashboarded, which will help track mk2bp progress.

Bug: 217764943
Test: manual; verified mk_metrics.pb is generated iff --mk-metrics is
specified, used printproto on result

Change-Id: I348994b4d7e3479d0dc04de0276b94a702aac95b