kvflowcontrol: surface per-replication stream metrics

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

There's a lack of metrics for every pair in kvflowcontrol, which can make it hard to diagnose exactly how writes are being being shaped. Log statements printed every 30s could miss token exhaustion.

Some notes from internal discussions:
- If we have a 10-node cluster, we’re adding 10*10 metrics at most, since most cluster setups are small.
- Look into adding more into the logs themselves. Maybe capture all streams that were blocked at any point in the last 30s, throughput achieved in that 30s delta, etc. So we have something historical.

Jira issue: CRDB-31711