Starlette middleware not respecting "capture_body" flag

[frnkvieira]

Hello :)

While investigating a bug regarding very slow uploads, we discovered two things about the Starlette middleware.

• It does not honor the "capture_body" flag by consuming the stream regardless the flag is turned on or not. It replaces the original stream with a mocked one, making the Starlette request.stream() feature useless because the request is already entirely in memory (using a lot of memory in case of file uploads). I think the following piece of code could check the flag and behave differently.

  apm-agent-python/elasticapm/contrib/starlette/__init__.py

  Lines 151 to 163 in 69beac4

  	body = b""
  	while True:
  	message = await receive()
  	if not message:
  	break
  	if message["type"] == "http.request":
  	b = message.get("body", b"")
  	if b:
  	body += b
  	if not message.get("more_body", False):
  	break
  	if message["type"] == "http.disconnect":
  	break

• The same block of code above uses byte concatenation instead of a byte array which is fine for very small buffers but prohibitively slow with anything bigger because of the quadratic behavior. Gigabyte-sized streams do not work at all.

I would happily send a pull request for that, but I would like some opinions first, especially after seeing this (#1528), which seems promising.

Anyway, Elastic APM is awesome. Thanks for the great work!

[basepi]

@frnkvieira Great catch! We would love to see a PR if you have time. Don't worry about waiting for #1528. :)

Keep us posted if you don't have time and I can submit a quick fix.