Add prometheus_endpoint_path for variables

What does this MR do?

We are adding a new type of variable called metric_label_values to the metrics dashboard YAML format (#214539 (closed)). This new variable type uses the label values of a Prometheus metric to populate the dropdown selector.

The frontend needs to make a Prometheus request in order to get the possible values of the variable.

Similar to how the backend adds a prometheus_endpoint_path key to the metrics in the dashboard YAML, this MR adds a prometheus_endpoint_path key to variables of type metric_label_values. The frontend will send a Prometheus request to this path, and use the response to populate the dropdown selector.

This MR:

1. Renames Stages::EndpointInserter to Stages::MetricEndpointInserter.
2. Creates a new class called Stages::VariableEndpointInserter which adds the prometheus_endpoint_path key to any metric_label_values variable type present in the dashboard being processed.
3. Adds an example of a metric_label_values variable to the sample_dashboard.yml spec fixture file.
4. Updates the dashboard JSON schema to allow metric_label_values variable types.

Issue: #214539 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
  Docs MR: !34762 (merged) to be merged after frontend MR (!34607 (merged))
• 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

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• [-] 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 tea