feat: add metrics tracers

[lszinv]

Description

This PR introduces a metrics factory to the codebase. It's a direct follow-up to the previously merged metrics exporter (see PR #2272). This factory is responsible for creating and managing the specific service metrics that will be instrumented throughout the code in subsequent changes.

This is a re-opening of #2311 with comments addressed, and rebased over main as the PR branch had gotten largely out of sync.

Impact

This change introduces new components but has no immediate impact as the factory is not called at this time.

Testing

Unit tests have been added to verify the correct functionality of the metrics factory and its interaction with the exporter. Additionally, manual verification confirmed that the factory and exporter correctly create and are capable of emitting metrics to Google Cloud Platform (GCP).

This PR introduces no breaking changes.

Additional Information

Future metrics work will involve:

• Developing a gRPC interceptor to capture and emit Google Front End (GFE) metrics.
• Integrating the created metrics into various parts of the codebase for actual instrumentation.

Checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated
• Appropriate comments were added, particularly in complex areas or places that require background
• No new warnings or issues will be generated from this change

[surbhigarg92]

getMeterProvider takes resourceAttributes now ?

[lszinv]

Removed the comment, but the intention is that the provider should be instantiated at spanner client creation (right before these metrics instruments are expected to be created), so the call here would just be retrieving it, and thus why the resourceAttributes argument isn't needed here.