Fix broken link in Prometheus exporter README. Fixes #4399

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Add missing Prometheus exporter documentation
+  ([#4485](https://github.com/open-telemetry/opentelemetry-python/pull/4485))
 - Fix user agent in OTLP HTTP metrics exporter
   ([#4475](https://github.com/open-telemetry/opentelemetry-python/pull/4475))
 - Improve performance of baggage operations

docs/exporter/prometheus/prometheus.rst

@@ -0,0 +1,54 @@
+OpenTelemetry Prometheus Exporter
+=================================
+
+.. automodule:: opentelemetry.exporter.prometheus
+    :members:
+    :undoc-members:
+    :show-inheritance:
+
+Installation
+------------
+
+The OpenTelemetry Prometheus Exporter package is available on PyPI::
+
+    pip install opentelemetry-exporter-prometheus
+
+Usage
+-----
+
+The Prometheus exporter starts an HTTP server that collects metrics and serializes them to 
+Prometheus text format on request::
+
+    from prometheus_client import start_http_server
+
+    from opentelemetry import metrics
+    from opentelemetry.exporter.prometheus import PrometheusMetricReader
+    from opentelemetry.sdk.metrics import MeterProvider
+    from opentelemetry.sdk.resources import SERVICE_NAME, Resource
+
+    # Service name is required for most backends
+    resource = Resource(attributes={
+        SERVICE_NAME: "your-service-name"
+    })
+
+    # Start Prometheus client
+    start_http_server(port=9464, addr="localhost")
+    # Initialize PrometheusMetricReader which pulls metrics from the SDK
+    # on-demand to respond to scrape requests
+    reader = PrometheusMetricReader()
+    provider = MeterProvider(resource=resource, metric_readers=[reader])
+    metrics.set_meter_provider(provider)
+
+Configuration
+-------------
+
+The following environment variables are supported:
+
+* ``OTEL_EXPORTER_PROMETHEUS_HOST`` (default: "localhost"): The host to bind to
+* ``OTEL_EXPORTER_PROMETHEUS_PORT`` (default: 9464): The port to bind to
+
+References
+----------
+
+* `Prometheus <https://prometheus.io/>`_
+* `OpenTelemetry Project <https://opentelemetry.io/>`_