Add a metric to measure the number of bytes written by certstore

[masih]

Measuring the size growth of metadata store in lotus is tricky. Because there are no direct datastore metrics for it, and even then there are none per datastore key prefix.

However this would be much easier to measure in F3, where we simply measure the number of bytes written out by certstore.

Add those metrics.

While at it add them for any other struct that writes to any ds, tagged by struct name or something similar.

[masih]

This is actually done as part of https://github.com/filecoin-project/go-f3/pull/587

We should have all the fine-grained metrics we need to determine the growth rate of certstore.