Add info about environment_filter variable

doc/operations/metrics/dashboards/variables.md

@@ -18,6 +18,7 @@ Queries that continue to use the old format will show no data.
 
 GitLab supports a limited set of [CI variables](../../../ci/variables/README.md) in the Prometheus query. This is particularly useful for identifying a specific environment, for example with `ci_environment_slug`. The supported variables are:
 
+- `environment_filter`
 - `ci_environment_slug`
 - `kube_namespace`
 - `ci_project_name`
@@ -29,6 +30,14 @@ GitLab supports a limited set of [CI variables](../../../ci/variables/README.md)
 NOTE: **Note:**
 Variables for Prometheus queries must be lowercase.
 
+### environment_filter
+
+`environment_filter` is automatically expanded to `container_name!="POD",environment="ENVIRONMENT_NAME"`
+where `ENVIRONMENT_NAME` is the name of the current environment.
+
+For example, a Prometheus query like `container_memory_usage_bytes{ {{environment_filter}} }`
+becomes `container_memory_usage_bytes{ container_name!="POD",environment="production" }`.
+
 ### __range
 
 The `__range` variable is useful in Prometheus

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

@@ -18,6 +18,8 @@ The [Prometheus service](../prometheus.md) must be enabled.
 
 NGINX server metrics are detected, which tracks the pages and content directly served by NGINX.
 
+[`environment_filter`](../../../../operations/metrics/dashboards/variables.md#environment_filter) is one of the predefined variables that metrics dashboards support.
+
 | Name | Query |
 | ---- | ----- |
 | Throughput (req/sec) | `sum(rate(nginx_server_requests{server_zone!="*", server_zone!="_", %{environment_filter}}[2m])) by (code)` |