Discourage the usage of labels with high cardinality

doc/development/prometheus_metrics.md

@@ -75,5 +75,10 @@ This section describes how to add new metrics for self-monitoring
 1. Select the appropriate name for your metric. Refer to the guidelines
    for [Prometheus metric names](https://prometheus.io/docs/practices/naming/#metric-names).
 1. Update the list of [GitLab Prometheus metrics](../administration/monitoring/prometheus/gitlab_metrics.md).
+1. Carefully choose what labels you want to add to your metric. Values with high cardinality,
+like `project_path`, or `project_id` are strongly discouraged because they can affect our services
+availability due to the fact that each set of labels is exposed as a new entry in the `/metrics` endpoint.
+For example, a histogram with 10 buckets and a label with 100 values would generate 1000
+entries in the export endpoint.
 1. Trigger the relevant page or code that records the new metric.
 1. Check that the new metric appears at `/-/metrics`.