metricsd: Fix logging initialization.

metricsd should only log to a single location (stderr or syslog):
* standard error if --logtostderr is set or metricsd runs in the
  foreground.
* syslog if --logtosyslog is set or metricsd is daemonized.

Bug: 25472752
Change-Id: Idb918714e1b6975d79682f22495a7ca708d4d97d