safekeeper: add metric for walreceiver queue depth

[erikgrinaker]

Problem

We don't have any observability for Safekeeper WAL receiver queues.

Resolves #9328.

Summary of changes

Add a safekeeper_wal_receiver_queue_depth histogram for queue depths per WAL receiver. Sampled once per Prometheus metrics interval (10 seconds).

Checklist before requesting a review

• [ ] I have performed a self-review of my code.
• [ ] If it is a core feature, I have added thorough tests.
• [ ] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• [ ] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• [ ] Do not forget to reformat commit message to not include the above checklist