Add OTLP over HTTP/protobuf export support

[AmreshSinha]

Introduces an --otlp-protocol CLI option (grpc or http-proto) to select the OTLP transport, and supports custom export headers via OTEL_EXPORTER_OTLP_HEADERS. Also updates the Python backend and documentation to use the selected protocol.

What does this PR do?

Fixes #911

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines.
• Did you write any new necessary tests? If applicable, did you include or update the insta snapshots?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.