feat: add connection retry and metrics

Retry failed proxy connections with a fresh random proxy on each
attempt (configurable via retries setting, proxy_source only).
Track connection metrics and log summary every 60s and on shutdown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USAGE.md

@@ -41,6 +41,7 @@ cp config/example.yaml config/s5p.yaml
 ```yaml
 listen: 127.0.0.1:1080
 timeout: 10
+retries: 3            # max attempts per connection (proxy_source only)
 log_level: info
 
 chain:
@@ -101,6 +102,40 @@ s5p -C socks5://127.0.0.1:9050 -S http://10.200.1.250:8081/proxies
 The API must return JSON: `{"proxies": [{"proto": "socks5", "proxy": "host:port"}, ...]}`.
 Entries with `null` proto are skipped.
 
+## Connection Retry
+
+When `proxy_source` is active, s5p retries failed connections with a different
+random proxy. Controlled by the `retries` setting (default: 3). Static-only
+chains do not retry (retrying the same chain is pointless).
+
+```yaml
+retries: 5   # try up to 5 different proxies per connection
+```
+
+```bash
+s5p -r 5 -C socks5://127.0.0.1:9050 -S http://api:8081/proxies
+```
+
+## Metrics
+
+s5p tracks connection metrics and logs a summary every 60 seconds and on
+shutdown:
+
+```
+metrics: conn=142 ok=98 fail=44 retries=88 active=3 in=1.2M out=4.5M up=0h05m12s
+```
+
+| Counter | Meaning |
+|---------|---------|
+| `conn` | Total incoming connections |
+| `ok` | Successfully connected + relayed |
+| `fail` | All retries exhausted |
+| `retries` | Total retry attempts |
+| `active` | Currently relaying |
+| `in` | Bytes client -> remote |
+| `out` | Bytes remote -> client |
+| `up` | Server uptime |
+
 ## Profiling
 
 ```bash