Permit to override default OTLP exporter headers

[xrmx]

Description

We would like being able to use a specific user agent on our distro in order to discriminate OTLP traffic from our distro and upstream. At the moment this is not possible because exporter always override the passed headers with the default ones. Currently the HTTP exporters sets content-type and user-agent while the grpc ones just user-agent.

It was pointed out that we may not even set grpc headers correctly but on the backend we get them just fine.

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?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox

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

[xrmx]

Wondering if instead of that we should make the default headers substituted via an entry point (or even make them a param for the init functions called by _initialize_components) so that we have a different mechanism that the user cannot easily override. This is because otherwise users setting OTEL_EXPORTER_OTLP_HEADERS will always override what distro sets.