[cmd/telemetrygen] improve test coverage of OTLP transport options

[axw]

Component(s)

cmd/telemetrygen

Describe the issue you're reporting

Coverage of the OTLP exporter flags is lacking, to the point that some flags have been found not to work at all. We should add functional test cases covering all flags. One notable gap is mTLS:

https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/35735/files/a474780b07bd677057b3ea12ae61b520d70c9493#r1797036485

[github-actions]

Pinging code owners:

• cmd/telemetrygen: @mx-psi @codeboten

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

Pinging code owners:

• cmd/telemetrygen: @mx-psi @codeboten

See Adding Labels via Comments if you do not have permissions to add labels yourself.