Reimplement: Add per-client queue length metrics

This issue tracks reimplementation of the work from stale PR #2754, which is being closed because it is too out of date to merge directly.

    Original PR: https://github.com/clockworklabs/SpacetimeDB/pull/2754
    Original author: @gefjon
    Original branch: `phoebe/metric-client-queue-length`
    Base branch: `master`

    ## Original PR summary

    # Description of Changes

This PR adds two new metrics, spacetime_client_connection_incoming_queue_length and spacetime_client_connection_outgoing_queue_length, which track the lengths of the per-client incoming and outgoing message queues. We expect to use these metrics in testing, incl. bot testing, but not to merge them, as we expect the cardinality of per-client metrics to be too high. I will open a separate PR with per-database versions of these metrics which we can merge.

    ## Follow-up

    - Reimplement this change in a fresh PR against current `master`.
    - Carry forward any still-relevant context from the original PR discussion and review.
    - Link the new implementation PR back to the original stale PR for historical context.

Start by reviewing stale PR #2754 and its discussion, then compare the metric instrumentation on the current master branch. Reimplement the still-relevant per-client incoming and outgoing queue length metrics in a fresh PR, preserving the named metric behavior and linking back to PR #2754.