update readme

README.md

@@ -9,6 +9,7 @@ Herald is a Gitea bot that performs automated AI-powered code reviews on pull re
 - Concurrent review processing with configurable parallelism
 - Graceful shutdown — in-progress reviews finish before the process exits
 - Error tracking via Sentry
+- Prometheus metrics endpoint for monitoring
 - Tiny memory footprint (~4MB) thanks to Rust
 
 ## Installation
@@ -34,6 +35,7 @@ Herald reads its configuration from environment variables (a `.env` file is supp
 | `GITEA_URL` | Base URL of your Gitea instance |
 | `GITEA_TOKEN` | Gitea API token |
 | `GITEA_TIMEOUT` | Gitea API request timeout in seconds |
+| `METRICS_BIND_ADDR` | *(optional)* Bind address for the Prometheus metrics endpoint (e.g. `0.0.0.0:9100`). If unset, the metrics exporter is disabled. |
 | `SENTRY_DSN` | *(optional)* Sentry DSN for error tracking |
 | `RUST_LOG` | *(optional)* Log level, defaults to `info` |
 
@@ -51,3 +53,40 @@ cargo run
 ```
 
 Copy `.env.example` to `.env` and fill in your values before running.
+
+## Metrics
+
+Herald optionally exposes a Prometheus metrics endpoint, useful for scraping with an [OpenTelemetry Collector](https://opentelemetry.io/docs/collector/) or any Prometheus-compatible scraper.
+
+Set `METRICS_BIND_ADDR` (e.g. `0.0.0.0:9100`) to enable it. The metrics are then available at `http://<host>:9100/metrics`.
+
+### Exposed metrics
+
+| Metric | Type | Description |
+|---|---|---|
+| `herald_webhooks_received_total` | counter | Total webhooks received (label: `event_type`) |
+| `herald_webhooks_duplicate_total` | counter | Webhooks rejected as duplicates (label: `event_type`) |
+| `herald_webhooks_channel_full_total` | counter | Webhooks dropped because the bot channel was full (label: `event_type`) |
+| `herald_bot_tasks_active` | gauge | Bot tasks currently in progress |
+| `herald_bot_tasks_completed_total` | counter | Bot tasks completed successfully (label: `event_type`) |
+| `herald_bot_tasks_failed_total` | counter | Bot tasks that failed (label: `event_type`) |
+| `herald_openrouter_cost_cents_total` | counter | Total OpenRouter cost in cents (divide by 100 for USD) |
+
+### OTel collector example
+
+```yaml
+receivers:
+  prometheus:
+    config:
+      scrape_configs:
+        - job_name: herald
+          scrape_interval: 15s
+          static_configs:
+            - targets: ["herald:9100"]
+
+service:
+  pipelines:
+    metrics:
+      receivers: [prometheus]
+      exporters: [otlp]
+```