Partial Config Update: event metric

Step 1. Determines if a metric needs to be updated
Step 2: performs the update. if the metric is replaced, it creates a new
metric. If the metric is preserved, we update the appropriate indices.
In both cases, the appropriate data structures that MetricsManager
relies on are updated.

Test: atest_statsd_test
Bug: 162322294
Change-Id: Iac24e02c70880bb8d0d42bd2da34acf84161ba16