Add development documentation

doc/development/README.md

@@ -47,6 +47,7 @@ description: 'Learn how to contribute to GitLab.'
 - [How to dump production data to staging](db_dump.md)
 - [Working with the GitHub importer](github_importer.md)
 - [Working with Merge Request diffs](diffs.md)
+- [Prometheus metrics](prometheus_metrics.md)
 
 ## Performance guides
 

doc/development/prometheus_metrics.md

+# 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
+```

doc/user/project/integrations/prometheus_library/metrics.md

@@ -17,48 +17,3 @@ GitLab retrieves performance data from the configured Prometheus server, and att
 
 In order to isolate and only display relevant metrics for a given environment, GitLab needs a method to detect which labels are associated. To do that,
 GitLab uses the defined queries and fills in the environment specific variables. Typically this involves looking for the [$CI_ENVIRONMENT_SLUG](../../../../ci/variables/README.md#predefined-variables-environment-variables), but may also include other information such as the project's Kubernetes namespace. Each search query is defined in the [exporter specific documentation](#prometheus-metrics-library).
-
-## 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 metrics is to have each query to have 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
-```