Document existing functionality for YAML dynamic labels

What does this MR do?

Currently, the label property in dashboard YAML files supports adding variables to it tied to the labels of the time series returned by the Prometheus queries. This feature in undocumented.

metrics:
  - id: metric_of_ages_02
    query_range: http_requests_total
    label: "Instance: {{instance}}, method: {{method}}"          
    unit: "count"

The example above renders the labels of the legend items dynamically, by looking for the interpolated variable names {{instance}} and {{method}} among the labels of the timeseries returned by the Prometheus query, and replacing them in the label with their respective values.

metrics:
  - id: metric_of_ages_02
    query_range: http_requests_total
    label: "Method"          
    unit: "count"

This example does the same, except it's a shorthand for using only one timeseries label (in this case, method, which is derived from changing the value to lowercase and replacing any inside spaces with "_").

Related issues

• #201998 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.