docs(fastapi): add usage example and setup order guidance

instrumentation/opentelemetry-instrumentation-fastapi/README.rst

@@ -1,15 +1,12 @@
 OpenTelemetry FastAPI Instrumentation
 =======================================
-
 |pypi|
 
 .. |pypi| image:: https://badge.fury.io/py/opentelemetry-instrumentation-fastapi.svg
    :target: https://pypi.org/project/opentelemetry-instrumentation-fastapi/
 
-
 This library provides automatic and manual instrumentation of FastAPI web frameworks,
 instrumenting http requests served by applications utilizing the framework.
-
 auto-instrumentation using the opentelemetry-instrumentation package is also supported.
 
 Installation
@@ -19,8 +16,57 @@ Installation
 
     pip install opentelemetry-instrumentation-fastapi
 
+Usage
+-----
+
+Configure your ``TracerProvider`` and call ``FastAPIInstrumentor.instrument_app()``
+**before** defining routes or handling requests. Instrumentation applied after route
+registration will not capture spans for those routes.
+
+.. code-block:: python
+
+    from fastapi import FastAPI
+    from opentelemetry import trace
+    from opentelemetry.sdk.trace import TracerProvider
+    from opentelemetry.sdk.trace.export import BatchSpanProcessor
+    from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter
+    from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
+
+    # 1. Configure the TracerProvider first
+    provider = TracerProvider()
+    provider.add_span_processor(BatchSpanProcessor(OTLPSpanExporter()))
+    trace.set_tracer_provider(provider)
+
+    # 2. Create the FastAPI app
+    app = FastAPI()
+
+    # 3. Instrument the app before defining routes
+    FastAPIInstrumentor.instrument_app(app)
+
+    # 4. Define routes after instrumentation
+    @app.get("/")
+    async def root():
+        return {"message": "Hello World"}
+
+.. note::
+
+    ``instrument_app()`` must be called after the ``FastAPI`` instance is created
+    but before the application starts serving requests. Calling it after routes are
+    registered is supported, but calling it after the application has started
+    handling requests may result in missing spans.
+
+Exclude URLs
+------------
+
+To exclude certain URLs from instrumentation, pass a comma-separated string of
+regex patterns via the ``excluded_urls`` parameter:
+
+.. code-block:: python
+
+    FastAPIInstrumentor.instrument_app(app, excluded_urls="/healthz,/metrics")
+
 References
 ----------
 
 * `OpenTelemetry Project <https://opentelemetry.io/>`_
-* `OpenTelemetry Python Examples <https://github.com/open-telemetry/opentelemetry-python/tree/main/docs/examples>`_
+* `OpenTelemetry Python Examples <https://github.com/open-telemetry/opentelemetry-python/tree/main/docs/examples>`_