sqlliveness/slstorage: record and log when deleting a session

[ajwerner]

There are two paths which delete sessions: a background job and on demand
when there's a cache miss. The second one, added later, and more commonly hit
did not log or record metrics.

Release note (ops change): Record a log event and counter increment when
removing an expired session.

[cockroach-teamcity]

This change is 

[knz]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @rimadeodhar)

[ajwerner]

TFTR!

bors r=knz

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 69a935a to blathers/backport-release-20.2-68476: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 20.2.x failed. See errors above.

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