spanconfig: add component metrics

[irfansharif]

Is your feature request related to a problem? Please describe.

The following metrics are useful to surface from the span configs infrastructure:

• number of KVSubscriber restarts due to internal errors
• number of KVAccessor invocations, number of stored spans, how long operations take (even if just a counter of total duration)
• count clock-past-lease-expiration errs (added for spanconfig: ensure safety without mutual exclusion in the reconciliation job #73789)
• per-store metrics on how many replicas each tenant has

Jira issue: CRDB-15834

[ajwerner]

Here's some more ideas:

• Reconciler:
  • Reconciliations started / completed (full vs incremental)
  • Total time spent in reconciliation (can divide for average times)
  • Num RPCs sent, num deletes, num upserts
• Server side
  • RPCs received
  • statements issued
  • total statement time
  • errors

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!