feat(trace): implement span start/end metrics

[anuraaga]

Description

I am helping to implement SDK internal metrics

https://opentelemetry.io/docs/specs/semconv/otel/sdk-metrics/

I have started with the most basic of them, started and ended spans. This sets up some test infrastructure as well and will be good to confirm before implementing more.

For reference, similar change in
Java: open-telemetry/opentelemetry-java#7895
JS: open-telemetry/opentelemetry-js#6213

/cc @xrmx to help with review

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

• Unit tests

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

[anuraaga]

Hmm - this change may be hard since many contrib tests seem to verify metric count itself, not just the metrics from the instrumentation. Any ideas how to proceed?