Import common metrics into database.

This MR backports PrometheusMetric model to CE
and adds: common, identifier to figure out what kind of metric is used.

1 files changed, 41 insertions, 2 deletions

diff --git a/doc/user/project/integrations/prometheus_library/metrics.md b/doc/user/project/integrations/prometheus_library/metrics.md
index 96a22316265..1e0f9d4e249 100644
--- a/doc/user/project/integrations/prometheus_library/metrics.md
+++ b/doc/user/project/integrations/prometheus_library/metrics.md
@@ -20,6 +20,45 @@ GitLab uses the defined queries and fills in the environment specific variables.
 
 ## 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 `additional_metrics.yml`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/prometheus/additional_metrics.yml) file.
+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.
 
-> Note: The library is only for monitoring public, common, system services which all customers can benefit from. Support for monitoring [customer proprietary metrics](https://gitlab.com/gitlab-org/gitlab-ee/issues/2273) will be added in a subsequent release.
+### 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
+```