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+# Working with Prometheus Metrics
+
+## Adding to the library
+
+We strive to support the 2-4 most important metrics for each common system service that supports Prometheus. If you are looking for support for a particular exporter which has not yet been added to the library, additions can be made [to the `common_metrics.yml`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/prometheus/common_metrics.yml) file.
+
+### Query identifier
+
+The requirement for adding a new metrics is to make each query to have an unique identifier.
+Identifier is used to update the metric later when changed.
+
+```yaml
+- group: Response metrics (NGINX Ingress)
+  metrics:
+  - title: "Throughput"
+    y_label: "Requests / Sec"
+    queries:
+    - id: response_metrics_nginx_ingress_throughput_status_code
+      query_range: 'sum(rate(nginx_upstream_responses_total{upstream=~"%{kube_namespace}-%{ci_environment_slug}-.*"}[2m])) by (status_code)'
+      unit: req / sec
+      label: Status Code
+```
+
+### Update existing metrics
+
+After you add or change existing _common_ metric you have to create a new database migration that will query and update all existing metrics.
+
+**Note: If a query metric (which is identified by `id:`) is removed it will not be removed from database by default.**
+**You might want to add additional database migration that makes a decision what to do with removed one.**
+**For example: you might be interested in migrating all dependent data to a different metric.**
+
+```ruby
+class ImportCommonMetrics < ActiveRecord::Migration
+  require_relative '../importers/common_metrics_importer.rb'
+
+  DOWNTIME = false
+
+  def up
+    Importers::CommonMetricsImporter.new.execute
+  end
+
+  def down
+    # no-op
+  end
+end
+```