ext/requests - apply custom attributes callback

[nirsky]

This PR implements passing an optional span_callback callback when instrumenting requests.
The callback will be called just before returning the result and will be invoked with the span and the result (type of requests.Response).
This mimics the same functionality as in js http plugin.

Example:

import requests
from opentelemetry.ext.requests import RequestsInstrumentor
from opentelemetry.sdk.trace.export import Span

def apply(span: Span, result: requests.Response):
    span.set_attribute("http.request.body", result.request.body)

RequestsInstrumentor().instrument(
    span_callback=apply
)

Please LMK if and what docs need to be updated.

[toumorokoshi]

Thanks! The code looks good, just want to discuss the naming of the argument and make sure it's succinct.

[mauriciovasquezbernal]

LGTM, some minor comments.

I second @toumorokoshi about the callback name, it could be more generic. Approving now assuming you'll handle it :)

[toumorokoshi]

Great, thanks!