Fix xds client active wait time metric also add more logs.

[bohhyang]

Summary

Fixes a few issues in xds client metrics so that:

1. when initial data has not been received (for wildcard subscriber, could be haven't received the last batch), reconnections won't reset subscribe time and the subscriber state (still in INIT_PENDING), so that the wait time and server latency metrics can be tracked with the initial subscribe time.
2. when IRV enabled, after reconnect, receiving no new update since existing data is up-to-date, the active wait time metric is 0.
3. when IRV is disabled, receiving the same data after reconnection will set the subscribers to FETCHED state (from PENDING_AFTER_RECONNECT), so the wait time metric is 0.

Testing Done

QEI deploy indis-canary, verified under both IRV enabled and disabled cases, the logs and metrics are as expected.

[ZoabKapoor]

Nice, this looks right to me and the added tests are very thorough, thanks for fixing this!

It looks like you'll need to re-sync master and bump the version again before merging though