chore: documentation (#76)

* chore: update docstring example to use python code block

* docs: add documentation

* feat: add docs build + fix other workdlows

* fix: add missing return type

docs/module/metrics.md

@@ -0,0 +1,90 @@
+# Metrics
+
+Prometheus metrics integration with a decorator-based registry and multi-process support.
+
+## Installation
+
+=== "uv"
+    ``` bash
+    uv add "fastapi-toolsets[metrics]"
+    ```
+
+=== "pip"
+    ``` bash
+    pip install "fastapi-toolsets[metrics]"
+    ```
+
+## Overview
+
+The `metrics` module provides a [`MetricsRegistry`](../reference/metrics.md#fastapi_toolsets.metrics.registry.MetricsRegistry) to declare Prometheus metrics with decorators, and an [`init_metrics`](../reference/metrics.md#fastapi_toolsets.metrics.handler.init_metrics) function to mount a `/metrics` endpoint on your FastAPI app.
+
+## Setup
+
+```python
+from fastapi import FastAPI
+from fastapi_toolsets.metrics import MetricsRegistry, init_metrics
+
+app = FastAPI()
+metrics = MetricsRegistry()
+
+init_metrics(app, registry=metrics)
+```
+
+This mounts the `/metrics` endpoint that Prometheus can scrape.
+
+## Declaring metrics
+
+### Providers
+
+Providers are called once at startup and register metrics that are updated externally (e.g. counters, histograms):
+
+```python
+from prometheus_client import Counter, Histogram
+
+@metrics.register
+def http_requests():
+    return Counter("http_requests_total", "Total HTTP requests", ["method", "status"])
+
+@metrics.register
+def request_duration():
+    return Histogram("request_duration_seconds", "Request duration")
+```
+
+### Collectors
+
+Collectors are called on every scrape. Use them for metrics that reflect current state (e.g. gauges):
+
+```python
+@metrics.register(collect=True)
+def queue_depth():
+    gauge = Gauge("queue_depth", "Current queue depth")
+    gauge.set(get_current_queue_depth())
+```
+
+## Merging registries
+
+Split metrics definitions across modules and merge them:
+
+```python
+from myapp.metrics.http import http_metrics
+from myapp.metrics.db import db_metrics
+
+metrics = MetricsRegistry()
+metrics.include_registry(http_metrics)
+metrics.include_registry(db_metrics)
+```
+
+## Multi-process mode
+
+Multi-process support is enabled automatically when the `PROMETHEUS_MULTIPROC_DIR` environment variable is set. No code changes are required.
+
+!!! warning "Environment variable name"
+    The correct variable is `PROMETHEUS_MULTIPROC_DIR` (not `PROMETHEUS_MULTIPROCESS_DIR`).
+
+```bash
+export PROMETHEUS_MULTIPROC_DIR=/tmp/prometheus
+```
+
+---
+
+[:material-api: API Reference](../reference/metrics.md)