Improve prometheus queries used in Pod health dashboard

What does this MR do?

Improve the Prometheus queries used in the new K8s Pod health dashboard.

The CPU throttling chart will only show data for pods whose CPU has been throttled.

The charts don't change much, except for the new CPU throttling chart, so I'm not sure if a changelog is needed. Also, this dashboard was added in this milestone (13.3) itself, so there is already a changelog for this dashboard.

To view on GDK:

1. Attach a cluster to your project.
2. Install Prometheus managed app into the cluster.
3. Create an environment if the project doesn't have one.
4. Open Operations > Metrics and choose the "K8spPod health" from the selector.

Issue: #218649 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team