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/blob/master/config/prometheus/common_metrics.yml) file.
### Query identifier
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:
```yaml
- group: Response metrics (NGINX Ingress)
metrics:
- title: "Throughput"
y_axis:
name: "Requests / Sec"
format: "number"
precision: 2
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 an existing common metric, you must [re-run the import script](../administration/raketasks/maintenance.md#import-common-metrics) that will query and update all existing metrics.
Or, you can create a database migration:
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.
```ruby
class ImportCommonMetrics < ActiveRecord::Migration[4.2]
include Gitlab::Database::MigrationHelpers
DOWNTIME = false
def up
::Gitlab::DatabaseImporters::CommonMetrics::Importer.new.execute
end
def down
# no-op
end
end
```
## GitLab Prometheus metrics
GitLab provides [Prometheus metrics](../administration/monitoring/prometheus/gitlab_metrics.md)
to monitor itself.
### Adding a new metric
This section describes how to add new metrics for self-monitoring
([example](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/15440)).
1. Select the [type of metric](https://gitlab.com/gitlab-org/prometheus-client-mmap#metrics):
- `Gitlab::Metrics.counter`
- `Gitlab::Metrics.gauge`
- `Gitlab::Metrics.histogram`
- `Gitlab::Metrics.summary`
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. Trigger the relevant page/code that will record the new metric.
1. Check that the new metric appears at `/-/metrics`.