📝 Add annotations to HTTP middleware example (#11530)

Co-authored-by: Sofie Van Landeghem <svlandeg@users.noreply.github.com>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

fastapi/applications.py

@@ -4515,14 +4515,17 @@ class FastAPI(Starlette):
 
         ```python
         import time
+        from typing import Awaitable, Callable
 
-        from fastapi import FastAPI, Request
+        from fastapi import FastAPI, Request, Response
 
         app = FastAPI()
 
 
         @app.middleware("http")
-        async def add_process_time_header(request: Request, call_next):
+        async def add_process_time_header(
+            request: Request, call_next: Callable[[Request], Awaitable[Response]]
+        ) -> Response:
             start_time = time.time()
             response = await call_next(request)
             process_time = time.time() - start_time