feat: Add configurable trace metrics reporter

[arielvalentin]

These changes make it possible for users to inject a custom trace metrics reporter as part of the configuration block.

• Adds the OTLP Exporter as a test dependency to ensure default metrics reporters are injected by the configurator
• Disables the BSP reporter thread by default to avoid spawning threads for each new instance of the BatchSpanExporter created in test cases

[arielvalentin]

ping @open-telemetry/ruby-maintainers @open-telemetry/ruby-approvers

[arielvalentin]

@fbogsany please review again at your leisure.

[arielvalentin]

ping @open-telemetry/ruby-maintainers

[arielvalentin]

Ping @open-telemetry/ruby-approvers

[arielvalentin]

ah, the ole 1yr+ PR. you're a determined person and i apologize for the delays.

No apologies necessary. I let this PR linger until about 2 weeks ago... right before the holidays.

the test failures look related to truffle ruby. i hope that project is doing ok ❤️ . I think this still lgtm, re-approving, do we need to fix those CI errors before merging this tho (not sure so asking, dont have an opinion here).

I am not sure I understand the SimpleCov errors there for Truffle. I am not sure how my changes would have broken that.

also, just making sure i understand correctly, the small change in behavior here for ppl not supplying their own metrics reporter is that we now supply the no-op metrics reporter class as a default, right? I think that's fine but just stating that for my own confidence here since it's been awhile and i'm extremely rusty wrt this repo

Correct. The default metrics reporter is a no-op