Document FastAPI instrumentation setup

.changelog/4555.added

@@ -0,0 +1 @@
+`opentelemetry-instrumentation-fastapi`, `opentelemetry-instrumentation`: update docs

docs/instrumentation/base/instrumentation.rst

@@ -1,5 +1,8 @@
-OpenTelemetry Python Instrumentor
-=================================
+.. include:: ../../../opentelemetry-instrumentation/README.rst
+    :end-before: References
+
+API
+---
 
 .. automodule:: opentelemetry.instrumentation
     :members:

instrumentation/opentelemetry-instrumentation-fastapi/src/opentelemetry/instrumentation/fastapi/__init__.py

@@ -18,6 +18,39 @@ async def foobar():
 
     FastAPIInstrumentor.instrument_app(app)
 
+Auto-instrumentation
+********************
+
+FastAPI can also be instrumented without code changes by using the
+``opentelemetry-instrument`` command from the
+`opentelemetry-instrumentation <https://pypi.org/project/opentelemetry-instrumentation/>`_
+package. For installation, runtime command, exporter configuration, and
+environment variables, see
+`Python zero-code instrumentation <https://opentelemetry.io/docs/zero-code/python/>`_.
+Install ``opentelemetry-instrumentation-fastapi`` in the same environment so
+the FastAPI integration is available to auto-instrumentation. FastAPI-specific
+options, such as excluded URLs and HTTP header capture, are documented below.
+
+Trace propagation
+*****************
+
+FastAPI instrumentation uses the underlying ASGI instrumentation, which extracts
+incoming context from request headers using the configured global OpenTelemetry
+propagator. WebSocket connections are instrumented as ASGI ``websocket`` scopes,
+with context extracted from the handshake headers. Propagation data inside
+WebSocket message payloads is not parsed automatically. For more information,
+see `Python context propagation <https://opentelemetry.io/docs/languages/python/propagation/>`_
+and `Python distro configuration <https://opentelemetry.io/docs/languages/python/distro/>`_.
+
+Logs
+****
+
+FastAPI instrumentation does not convert Python ``logging`` records to span
+events. For zero-code log export, see
+`Python logs auto-instrumentation <https://opentelemetry.io/docs/zero-code/python/logs-example/>`_.
+Use ``span.add_event(...)`` for application data that should be recorded as span
+events.
+
 Configuration
 -------------