contrib/starlette: take into account body reading time

Create the transaction always before the body is read to avoid
discrepancies between cases where the capture_body config on or off.

Fix #1948

Author: xrmx

elasticapm/contrib/starlette/__init__.py

@@ -36,6 +36,7 @@
 from typing import Dict, Optional
 
 import starlette
+from starlette.datastructures import Headers
 from starlette.requests import Request
 from starlette.routing import Match, Mount
 from starlette.types import ASGIApp, Message
@@ -151,6 +152,10 @@ async def wrapped_send(message) -> None:
         _mocked_receive = None
         _request_receive = None
 
+        # begin the transaction before capturing the body to get that time accounted
+        trace_parent = TraceParent.from_headers(dict(Headers(scope=scope)))
+        self.client.begin_transaction("request", trace_parent=trace_parent)
+
         if self.client.config.capture_body != "off":
 
             # When we consume the body from receive, we replace the streaming
@@ -234,9 +239,6 @@ async def _request_started(self, request: Request) -> None:
         if self.client.config.capture_body != "off":
             await get_body(request)
 
-        trace_parent = TraceParent.from_headers(dict(request.headers))
-        self.client.begin_transaction("request", trace_parent=trace_parent)
-
         await set_context(lambda: get_data_from_request(request, self.client.config, constants.TRANSACTION), "request")
         transaction_name = self.get_route_name(request) or request.url.path
         elasticapm.set_transaction_name("{} {}".format(request.method, transaction_name), override=False)