Merge branch 'mc/docs/metrics-reports-docs' into 'master'

Metrics reports documentation is incomplete

Closes gitlab-ee#11942

See merge request gitlab-org/gitlab-ce!29345

doc/ci/metrics_reports.md

@@ -28,6 +28,11 @@ Consider the following examples of data that can utilize Metrics Reports:
 
 Metrics are read from the metrics report (default: `metrics.txt`). They are parsed and displayed in the MR widget.
 
+All values are considered strings and string compare is used to find differences between the latest available `metrics` artifact from:
+
+- `master`
+- The feature branch
+
 ## How to set it up
 
 Add a job that creates a [metrics report](yaml/README.md#artifactsreportsmetrics-premium) (default filename: `metrics.txt`). The file should conform to the [OpenMetrics](https://openmetrics.io/) format.