Django: Capture custom request/response headers

[sanketmehta28]

Description

This PR https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/semantic_conventions/http.md for django framework

Fixes #913

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Added unit tests separately for request and response headers for ASGI and WSGI applications. Also tested it manually by capturing the request and response headers from a instrumented django app for both ASGI and WSGI application

Same test cases for both ASGI/WSGI application

• Test A: check for request header which is set in OTEL_INSTRUMENTATION_HTTP_CAPTURE_HEADERS_SERVER_REQUEST env variable, is present in the span as attribute
• Test B: check for request header which is NOT set in OTEL_INSTRUMENTATION_HTTP_CAPTURE_HEADERS_SERVER_REQUEST env variable, is NOT present in the span as attribute
• Test C: check for response header which is set in OTEL_INSTRUMENTATION_HTTP_CAPTURE_HEADERS_SERVER_RESPONSE env variable, is present in the span as attribute
• Test D: check for response header which is NOT set in OTEL_INSTRUMENTATION_HTTP_CAPTURE_HEADERS_SERVER_RESPONSE env variable, is NOT present in the span as attribute

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[srikanthccv]

Please update the title

[sanketmehta28]

Please update the title

done

[sanketmehta28]

Hi @open-telemetry/python-approvers : please do review this PR

[lzchen]

I don't believe you have to explain this reasoning behind this, so I would remove the comments.