opentelemetry-exporter-otlp-proto-http: Update exporter to use new HTTP transport package

[herin049]

Description

This PR updates the opentelemetry-exporter-otlp-proto-http package to switch to using the abstractions/utilities added in the opentelemetry-exporter-http-transport package. Notably, this involves switching to the shared OTLPHTTPClient client (reducing code duplication between exporters), and adding an optional, private _transport argument that accepts an arbitrary transport implementation. These changes also switch to using urllib3 as the default transport, unless the user specifies otherwise, either by directly passing a requests Session, initializing the _OTEL_PYTHON_EXPORTER_OTLP_HTTP_CREDENTIAL_PROVIDER environment variable or directly passing a custom transport via the _transport keyword argument.

Fixes #3439, #4171, #2990

Depends on #5194

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

uv run tox -e py314-test-opentelemetry-exporter-otlp-proto-http

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

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