reduce number of loops over request headers from 2 to 1 (#54326)

`__getitem__` on headers is a O(n) operation. Optimize the code to cut
out the extra loop.

---------

Signed-off-by: abrar <[email protected]>

Author: abrarsheikh

python/ray/serve/_private/http_util.py

@@ -657,11 +657,11 @@ def __init__(self, app: ASGIApp):
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send):
         headers = MutableHeaders(scope=scope)
-        if SERVE_HTTP_REQUEST_ID_HEADER not in headers:
+        request_id = headers.get(SERVE_HTTP_REQUEST_ID_HEADER)
+
+        if request_id is None:
             request_id = generate_request_id()
             headers.append(SERVE_HTTP_REQUEST_ID_HEADER, request_id)
-        elif SERVE_HTTP_REQUEST_ID_HEADER in headers:
-            request_id = headers[SERVE_HTTP_REQUEST_ID_HEADER]
 
         async def send_with_request_id(message: Message):
             if message["type"] == "http.response.start":