Add get_metric_schema for GKE resource schemas

[erain]

Summary

• expose get_metric_schema for GKE monitored resource schemas used in metrics queries
• add schema docs for core Kubernetes/GKE resource types (cluster, node, node pool, pod, container, control plane component, service, scale, entity)
• add unit test coverage for schema lookup

Details

• validate supported resource types and map to embedded schema markdowns
• document resource labels and example MQL filters based on the Monitoring resources catalog

References

• https://cloud.google.com/monitoring/api/resources

Testing

• go test ./pkg/tools/monitoring

[bradhoekstra]

/assign @juli4n

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: erain

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [erain]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[erain]

I am blocking this PR until I do a full testing of all the sample queries. I will attach the test results as comments.

[GinnyJI]

Removing reviewers because this pr has been in draft stage for 4 months. Feel free to add reviewers back once this pr is ready.