KIP-1130 — Add metrics indicating the connection count exceeds

Discussion BrokerMetrics

Adds two new broker metrics — `waiting-connection` (gauge of connections queued for an available slot) and `connection-latency` (histogram of connection wait time) — per listener at the Acceptor level. When clients hit broker connection limits or throttling, they receive only a timeout exception with no broker-side visibility into how many connections are backlogged or how long they wait.

Details