[WIP] Add WithMetricProvider for BatchSpanProcessor

[logan-stytch]

Description

This PR introduces a WithMetricProvider option for the BatchSpanProcessor, implementing the same metrics that are currently provided by opentelemetry-java.

I roughly followed the comment in #2547, but so far only added support for the metrics currently supported in Java. I'm open to doing more, but felt a bit unsure especially of how to create a "new type that is the union of both Options" (which was suggested in the previous thread).

Testing

I haven't added any new tests yet. I suspect I probably want to add some sort of test in batch_span_processor_test.go to validate the metric behavior, but I'm not sure what exactly that should look like yet.

Where I'd like guidance

• What sort of testing should I add?
• Would it be desirable to add more metrics now (like those listed in Trace SDK observability #2547)?
• Creating the gauge and counter can return an error -- I wasn't sure how to handle this, so I added a panic for now, but I'm almost certain that's not the preferred approach.

[linux-foundation-easycla]

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• ✅ login: logan-stytch / name: Logan Gore (a73bdbc, c0b44a6)