[FEATURE REQ][Thin client]: HTTP/2 connection-level metrics in diagnostics — pool health, per-connection stats, and lifecycle events

## Summary

Thin client diagnostics already capture per-request context: `endpoint` (shows `:10250/` thin client URL), `parentChannelId`, `channelId`, and `isHttp2`. What's missing is connection-level metrics — pool health, per-connection lifecycle stats, and Micrometer integration for monitoring.

## Already Done

- `endpoint` in `GatewayStatistics` points to thin client URL (`:10250/`)
- `parentChannelId` / `channelId` identify HTTP/2 parent connection and stream
- `isHttp2: true` distinguishes thin client from gateway

## What's Missing — Connection-Level Metrics

## Acceptance Criteria

- [ ] HTTP/2 connection pool metrics surfaced (active connections, idle connections, pending acquires)
- [ ] Per-connection stats: requests served, errors, GOAWAY count, PING RTT
- [ ] Connection lifecycle events observable (created, evicted, closed, recycled)