Ignore non-existing metrics when uninstalling Prometheus

What does this MR do?

Both Prometheus and Knative try to delete the same metrics when uninstalling. If they're uninstalling at the same time, there might be a race condition where one tries to delete the metrics but the other has already deleted.

This MR adds a --ignore-not-found flag to the kubeclient command so that the deletion is ignored instead of failed in case the metric is not present.

This MR is important since we'll implement a feature that will uninstall all the GitLab managed apps at once: !16954 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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