gitlab-org--gitlab-foss/doc/development/prometheus_metrics.md

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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
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The requirement for adding a new metric is to make each query to have an unique identifier which is used to update the metric later when changed:
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```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.
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NOTE: **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.
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```ruby
class ImportCommonMetrics < ActiveRecord::Migration
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include Gitlab::Database::MigrationHelpers
require Rails.root.join('db/importers/common_metrics_importer.rb')
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DOWNTIME = false
def up
Importers::CommonMetricsImporter.new.execute
end
def down
# no-op
end
end
```