Add excluded_urls example for /

docs/integrations/fastapi.md

@@ -102,6 +102,44 @@ To avoid tracing certain URLs, you can specify a string of comma-separated regex
 - The environment variable `OTEL_PYTHON_FASTAPI_EXCLUDED_URLS`.
 - The environment variable `OTEL_PYTHON_EXCLUDED_URLS` (which will also apply to other instrumentation).
 
+!!! example
+    If you'd like to trace all URLs except the base `/` URL, you can use the following regex for `excluded_urls`: `^https?://[^/]+/$`
+
+    Breaking it down:
+
+    * `^` matches the start of the string
+    * `https?` matches `http` or `https`
+    * `://` matches `://`
+    * `[^/]+` matches one or more characters that are not `/` (this will be the host part of the URL)
+    * `/` matches `/`
+    * `$` matches the end of the string
+
+    So this regex will only match routes that have no path after the host.
+
+    This instrumentation might look like:
+
+    ```py
+    from fastapi import FastAPI
+
+    import logfire
+
+    app = FastAPI()
+
+    logfire.configure()
+    logfire.instrument_fastapi(app, excluded_urls='^https?://[^/]+/$')
+
+    if __name__ == '__main__':
+        import uvicorn
+
+        uvicorn.run(app)
+    ```
+
+    If you visit http://127.0.0.1:8000/, that matches the above regex, so no span will be sent to logfire.
+    If you visit http://127.0.0.1:8000/hello/ (or any other endpoing that's not `/`, for that matter), a span will be sent to logfire.
+
+    Note that under the hood, the `opentelemetry` library is using `re.search` (not `re.match` or `re.fullmatch`) to check for a match between the route and the `excluded_urls` regex, which is why we need to include the `^` at the start and `$` at the end of the regex.
+
+
 ## Capturing request and response headers
 <!-- note that this section is duplicated for different frameworks but with slightly different links -->