Pool exhaustion was invisible until checkouts timed out. Expose
deadpool's occupancy from the Rust pool and publish it as gauges per
pool (max connections, open connections, idle connections, waiting
checkouts), plus a histogram of the time a checkout waits for a
connection.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ULdieJyQoHNzeHq42H2M1p