OplogToRedis: Improve metrics handling for parallelism

[alex-goodisman]

Collection of a few metrics improvements:

• staleness metric is now on the publisher side in addition to the tail side
• added an additional staleness metric at the handoff between read and write
• staleness metrics now sharded by write parallelism
• lastprocessedtimestamp now computed separately per write parallelism, and the minimum is taken
• use proper coroutine variable reference when spinning up parallel coroutines in main to guarantee correct logs