docs_src/middleware/tutorial002.py: new tutorial source code added

docs_src/middleware/tutorial002.py

@@ -0,0 +1,22 @@
+import time
+
+from fastapi import FastAPI, Request, Response
+from starlette.middleware.base import BaseHTTPMiddleware
+
+
+class ProcessTimeHeaderMiddleware(BaseHTTPMiddleware):
+    async def dispatch(self, request: Request, call_next):
+        start_time = time.perf_counter()
+        response: Response = await call_next(request)
+        process_time = time.perf_counter() - start_time
+        response.headers["X-Process-Time"] = str(process_time)
+        return response
+
+
+app = FastAPI()
+app.add_middleware(ProcessTimeHeaderMiddleware)
+
+
+@app.get("/")
+def hello():
+    return {"hello": "world"}