output: add metrics for number of stored chunks in secondary

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
This PR will add metrics for number of stored chunks in secondary.
This metrics can be used to determine whether recovery is required from secondary if failure occurs.

Docs Changes:

Release Note:

[kenhys]

LGTM.

[daipom]

Hmm, we may need to work out the specifications in more detail.
For example...

• What this metrics mean exactly:
  • Is it really Number of stored chunks in secondary?
  • Maybe we can express it less misleadingly?
• The value increases in commit_write, but is it correct?
  • I'm concerned that the backup process calls commit_write as well.
  • What happens if the secondary uses delayed_commit?

For example, if a backup occurs in the secondary and it is also counted up as this metrics, it could be OK as a specification of this metrics.
What we need would be to sort out those specifications.