Support autoinstrumentation of vendored requests libraries

Is your feature request related to a problem?
I would like to be able to monitor the internals of the snowflake python connector that uses the requests library.

Describe the solution you'd like
Adding an optional argument to instrument() with the set of modules to instrument would be fine for me, as would adding an env var that lists what extra modules to try and instrument.

Describe alternatives you've considered
I have worked around this myself by copy and pasting instrumentation/opentelemetry-instrumentation-requests/src/opentelemetry/instrumentation/requests/init.py into my app and then changing the request imports to look like this:

from snowflake.connector.vendored.requests.models import PreparedRequest, Response
from snowflake.connector.vendored.requests.sessions import Session
from snowflake.connector.vendored.requests.structures import CaseInsensitiveDict

Obviously this is not very maintainable.

Start with instrumentation/opentelemetry-instrumentation-requests/src/opentelemetry/instrumentation/requests/init.py and compare its imports with the vendored snowflake.connector.vendored.requests modules listed in the issue. Determine how optional module selection or an environment variable would fit the existing instrumentation entry point; done means the Snowflake connector's vendored requests traffic can be instrumented without copying the instrumentation code.