Fail loudly on half-set metrics auth credentials (closes #205) (#216)

README.md

@@ -107,14 +107,26 @@ TTY detection, and security headers are always applied.
 | `DATA_DIR`              | Directory for all SQLite databases  | `/var/lib/webhooker` |
 | `DEBUG`                 | Enable debug logging                | `false`  |
 | `MAINTENANCE_MODE`      | Report `maintenanceMode: true` in the healthcheck JSON. It does not change how any request is served — no maintenance page exists | `false`  |
-| `METRICS_USERNAME`      | Basic auth username for `/metrics`  | `""`     |
-| `METRICS_PASSWORD`      | Basic auth password for `/metrics`  | `""`     |
+| `METRICS_USERNAME`      | Basic auth username for `/metrics`. Must be set together with `METRICS_PASSWORD`; one without the other fails startup | `""`     |
+| `METRICS_PASSWORD`      | Basic auth password for `/metrics`. Must be set together with `METRICS_USERNAME`; one without the other fails startup | `""`     |
 | `SENTRY_DSN`            | Sentry error reporting DSN          | `""`     |
 | `RETENTION_SWEEP_INTERVAL` | How often the retention reaper and archive sweeper run (Go duration, must be positive) | `1h` |
 | `SESSION_IDLE_TIMEOUT`  | Idle session timeout (Go duration)  | `24h`    |
 | `RECEIVER_RATE_LIMIT`   | Receiver requests/minute per IP per entrypoint (10x that per IP across the route) | `120` |
 | `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted (unset: all clients behind a proxy share one rate-limit bucket; a correct login password is never throttled either way) | `""` (none) |
 
+#### Metrics credentials
+
+`METRICS_USERNAME` and `METRICS_PASSWORD` are set together or not at
+all. With both set, `/metrics` is served behind basic auth. With
+neither set, the route is not registered and returns 404. With one set
+and the other empty or unset, the process refuses to start and exits
+non-zero with an error naming both variables — mounting the endpoint
+on the username alone would publish it behind a password that is the
+empty string, and quietly withholding it would deny an endpoint that
+was asked for. The `hasMetricsAuth` field in the startup log and the
+existence of the route are the same value, so they cannot disagree.
+
 #### Trusted proxies
 
 `TRUSTED_PROXIES` is a comma-separated list of CIDR blocks (a bare
@@ -1682,7 +1694,7 @@ abuse limit later; they are tracked as future work.
 
 | Method | Path       | Description |
 | ------ | ---------- | ----------- |
-| `GET`  | `/metrics` | Prometheus metrics, behind basic auth. The route is registered only when `METRICS_USERNAME` is set; otherwise it does not exist and returns 404 |
+| `GET`  | `/metrics` | Prometheus metrics, behind basic auth. The route is registered only when `METRICS_USERNAME` and `METRICS_PASSWORD` are both set; with neither set it does not exist and returns 404, and with only one set the process refuses to start |
 
 #### API (Planned)
 
@@ -1837,7 +1849,8 @@ Applied to all routes in this order:
    Permissions-Policy)
 3. **Logging** — Structured request logging (method, URL, status,
    latency, remote IP, user agent, request ID)
-4. **Metrics** — Prometheus HTTP metrics (if `METRICS_USERNAME` is set)
+4. **Metrics** — Prometheus HTTP metrics (if `METRICS_USERNAME` and
+   `METRICS_PASSWORD` are both set)
 5. **CORS** — Cross-origin resource sharing headers
 6. **Timeout** — 60-second request timeout
 7. **Recoverer** — Panic recovery: one `ERROR` record through
@@ -1869,8 +1882,9 @@ being read and without reaching CSRF, the route group's remaining
 middleware, or the handler. It is not rejected before *any* other
 middleware, though: the global entries listed above all run first, so
 such a request is still logged and given the security headers — and
-counted in the metrics, on a deployment where `METRICS_USERNAME` is
-set and the Metrics middleware is therefore registered at all. The
+counted in the metrics, on a deployment where the `/metrics`
+credentials are set and the Metrics middleware is therefore registered
+at all. The
 rejection itself is logged at `WARN` with the method, path and
 declared length. A chunked request, or
 one that lies about its length, is hard-capped by