allow statsd pull based on event trigger

Several restrictions:
1) This is only with GaugeMetric. We don't have use case for ValueMetric
to pull on event trigger.
2) trigger_event is set in the config. It can be generic atom matcher. But
we limit the number of atoms referenced in it to be 1. So we don't allow
multiple atoms to form a complex trigger.
3) This has to go with ALL_CONDITION_CHANGES sampling type.

+ also specify atom id of GaugeMetric output.

Bug: 111937835
Test: unit test
Change-Id: Ia15b1f209945f022edffb9ec5d673317d55d9e4f