Add Opt-in Metrics measurement

[Drew]

It would be nice to add some Metrics measurement capabilities.

We should look for:

• 'Opt in' behavior such that if metrics are disabled, the performance difference from before/after PR is negligible.
• Tracking of number of rows written in an operation (so that rows/sec may be measured)
• Time of operations:
  • read, write, delete

Questions:

• How do we want to let these metrics be observed?
  • Possibly an interface?
    • If people want Eventstream, they can wrap from that.
    • If people want direct, they can do that too.
• How granular do we want to be? DB RoundTrips are definitely on the menu, but should we also measure total operation time?
  • (example, when ReplayMessagesAsync has to make multiple round trips, do we report that as well as a separate metric?)
• Should we metric errors?
  • may be useful for scenarios where a RetryPolicy is defined.