Move metrics off reducer thread

[joshua-spacetime]

We're still recording transaction metrics on the main reducer thread. At one point this was happening outside of the tx lock, but it was probably reverted when we made the change to pin reducer threads.

[Centril]

At one point this was happening outside of the tx lock, but it was probably reverted when we made the change to pin reducer threads.

This part is not accurate. We are recording reducer / update metrics after releasing the tx lock.
The flamegraph does show that the reporting is happening in the same thread.
To make it not, we'd need to introduce a worker that reports the tx metrics.

[joshua-spacetime]

The flamegraph does show that the reporting is happening in the same thread.

Hence the title of the ticket.