Is your feature request related to a problem?
Python agent is not able to capture custom headers as of now. The specifications for capturing request/response headers is defined here http-request-response-header

Describe the solution you'd like
This feature aims to capture custom request/response headers with the help of environment variables.I think these environment variables should be same for all the frameworks (something like OTEL_PYTHON_CAPTURE_REQUEST_HEADERS, OTEL_PYTHON_CAPTURE_RESPONSE_HEADERS) and the request/response headers will only be added in case of a server span.

Currently, some frameworks like falcon and django captures custom request headers with the help of environment variables (i.e. OTEL_PYTHON_FALCON_TRACED_REQUEST_ATTRS etc). I suggest we should change the environment variable from being framework specific to be general for all the frameworks.

We can open multiple sub-issues to add support and test cases for each framework.

Additional context
With conditional server span creation #445 added we can ensure single server span creation and no two frameworks capturing the same headers.