Isolation of ProviderSession-Context Metrics

To continue what we began in RequestSession, this will begin to isolate
the provider session metric code to avoid core-code contamination. This
uses ideas discussed in the RequetsSessionMetric iteration.

Bug: 271135048
Test: Build and E2E test
Change-Id: Ife5b9d67a952534983d973584c1499be4551a8b9