Add metrics, request logs, and graceful shutdown

README.md

@@ -25,6 +25,8 @@ Tiny API that clones a Git repo and counts its non-empty lines of code.
   - Returns both snapshots and their total/per-language LOC delta.
 - `GET /health`
   - Health plus queue/cache stats.
+- `GET /metrics`
+  - Prometheus-compatible request, cache, scan, file, and line counters. Protected by the API key when one is configured.
 
 ## Auth
 
@@ -70,6 +72,12 @@ curl "http://localhost:3000/ssh/public-key?ssh_key=loc_via_git_ed25519"
 - The service also sweeps stale temp directories in case a process dies mid-scan.
 - Cache entries live in memory only and expire after `CACHE_TTL_MINUTES`.
 
+## Operations
+
+- Requests emit structured JSON logs with an opaque request ID, route, status, and duration. Query strings and API keys are never logged.
+- The service handles `SIGINT` and `SIGTERM`: it stops accepting new connections, clears its cleanup timer, and exits after active connections drain (or 10 seconds).
+- `/metrics` is compatible with Prometheus scraping. `/health` remains unauthenticated for container health checks.
+
 ## Counting rules
 
 - Only text files are counted.