Add custom metrics guide

Signed-off-by: Sunny <darkowlzz@protonmail.com>

content/en/flux/guides/custom-metrics.md

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+---
+title: "Custom Metrics"
+linkTitle: "Custom Metrics"
+description: "Configuring custom metrics for Flux resources."
+weight: 50
+---
+
+By default, the standard installation of Flux exports a specific set of metrics
+about the controllers and their inner workings that may not serve the needs of
+all the users. Some of these metrics are common across all the Flux controllers,
+and some are very specific to a few controllers. It's not feasible to
+add all the possible informational labels to these metrics, as that may increase
+the cardinality of the metrics. Since these metrics are about the operations of
+Flux, it may not be much of a benefit to add custom labels to these metrics,
+as these are more useful for the people administering and maintaining Flux. Most
+of the time, the users of Flux who interact with Flux through the Flux custom
+resources want to know about the resources they work with. For example, the
+state of GitRepositories and their branches or tag references. These metrics can
+be scraped by using [kube-state-metrics
+(KSM)](https://github.com/kubernetes/kube-state-metrics), which is part of the
+[kube-prometheus-stack](https://github.com/prometheus-operator/kube-prometheus).
+KSM can be configured to add custom labels to the resource metrics, for example,
+some value from the status of a resource or some arbitrary value like a team
+name, department name, etc.
+
+## Set up kube-state-metrics
+
+Kube-state-metrics can be installed along with the whole monitoring stack using
+kube-prometheus-stack. The
+[fluxcd/flux2-monitoring-example](https://github.com/fluxcd/flux2-monitoring-example)
+repository contains example configurations for deploying and configuring
+kube-prometheus-stack to monitor Flux. These configurations will be discussed in
+detail in the following sections to show how they can be customized. Refer to
+[Monitoring with Prometheus](monitoring.md) for detailed installation
+instructions.
+
+The [Helm chart values for
+kube-prometheus-stack](https://github.com/fluxcd/flux2-monitoring-example/blob/main/kube-prometheus-stack/release.yaml)
+configure KSM to run in `custom-resource-state-only` mode. In this state, KSM
+will not collect metrics for any of the Kubernetes core resources. The `rbac`
+section provides KSM access to list and watch Flux custom resources. If
+image-reflector-controller and image-automation-controllers are not used, the
+API group (`image.toolkit.fluxcd.io`) and resources (`imagerepositories`,
+`imagepolicies`, `imageupdateautomations`) can be removed. The
+`customResourceState` section configures how the Flux metrics are composed.
+
+Once deployed, KSM will start collecting the Flux resource metrics from the
+kube-apiserver and exporting them as configured.
+
+## Adding custom metrics
+
+The example `customResourceState` values used in the above setup add a metric
+called `gotk_resource_info` with labels `name`, `exported_namespace`, and
+`ready`.
+
+```yaml
+- name: "resource_info"
+  help: "The current state of a GitOps Toolkit resource."
+  each:
+    type: Info
+    info:
+    labelsFromPath:
+      name: [metadata, name]
+  labelsFromPath:
+    exported_namespace: [metadata, namespace]
+    ready: [status, conditions, "[type=Ready]", status]
+```
+
+This provides the current state of the Flux resources. It can be used to monitor
+the readiness of Flux resources. This can be edited to add more labels; more
+about that in the next section.
+
+Similarly, more custom metrics can be added by appending them to the `metrics`
+list. For example, to create a metric about the HelmRelease last applied
+revision, append the HelmRelease resource metrics section:
+
+```yaml
+...
+customResourceState:
+  config:
+    spec:
+      resources:
+        - groupVersionKind:
+            group: helm.toolkit.fluxcd.io
+            version: "v2beta1"
+            kind: HelmRelease
+          metricNamePrefix: gotk
+          metrics:
+            - name: "resource_info"
+              help: "The current state of a GitOps Toolkit resource."
+              each:
+                type: Info
+                info:
+                  labelsFromPath:
+                    name: [metadata, name]
+              labelsFromPath:
+                exported_namespace: [metadata, namespace]
+                ready: [status, conditions, "[type=Ready]", status]
+            - name: "helmrelease_version_info"
+              help: "The version information of helm release resource."
+              each:
+                type: Info
+                info:
+                  labelsFromPath:
+                    version: [status, lastAppliedRevision]
+              labelsFromPath:
+                name: [metadata, name]
+                exported_namespace: [metadata, namespace]
+                chartName: [spec, chart, spec, chart]
+...
+```
+
+In the above, `gotk_resource_info` and `gotk_helmrelease_version_info` metrics
+will be exported for HelmReleases.
+
+```
+# HELP gotk_resource_info The current state of a GitOps Toolkit resource.
+# TYPE gotk_resource_info info
+gotk_resource_info{customresource_group="helm.toolkit.fluxcd.io",customresource_kind="HelmRelease",customresource_version="v2beta1",exported_namespace="monitoring",name="kube-prometheus-stack",ready="True"} 1
+gotk_resource_info{customresource_group="helm.toolkit.fluxcd.io",customresource_kind="HelmRelease",customresource_version="v2beta1",exported_namespace="monitoring",name="loki-stack",ready="True"} 1
+# HELP gotk_helmrelease_version_info The version information of helm release resource.
+# TYPE gotk_helmrelease_version_info info
+gotk_helmrelease_version_info{chartName="kube-prometheus-stack",customresource_group="helm.toolkit.fluxcd.io",customresource_kind="HelmRelease",customresource_version="v2beta1",exported_namespace="monitoring",name="kube-prometheus-stack",version="48.3.1"} 1
+gotk_helmrelease_version_info{chartName="loki-stack",customresource_group="helm.toolkit.fluxcd.io",customresource_kind="HelmRelease",customresource_version="v2beta1",exported_namespace="monitoring",name="loki-stack",version="2.9.11"} 1
+```
+
+## Adding custom metric labels
+
+Custom labels can be added to metrics to create more meaningful monitoring
+metrics. For example, a common `ownedBy` label across all the resources in a
+cluster, `businessUnit` or `department` name from the labels of objects, etc.
+
+For example, if the GitRepository objects are labelled with `department`:
+
+```yaml
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: foo
+  namespace: bar
+  labels:
+    department: baz
+spec:
+  interval: 1h
+  ref:
+    branch: main
+  url: https://github.com/fluxcd/flux2-monitoring-example
+```
+
+The KSM `customResourceState` value can be configured to extract the
+department name, owned by team name and Git branch name, and include them as
+labels in the `gotk_resource_info` metric of GitRepositories.
+
+```yaml
+...
+customResourceState:
+  config:
+    spec:
+      resources:
+        - groupVersionKind:
+            group: source.toolkit.fluxcd.io
+            version: "v1"
+            kind: GitRepository
+          metricNamePrefix: gotk
+          commonLabels:
+            ownedBy: teamA
+          labelsFromPath:
+            department: [metadata, labels, department]
+          metrics:
+            - name: "resource_info"
+              help: "The current state of a GitOps Toolkit resource."
+              each:
+                type: Info
+                info:
+                  labelsFromPath:
+                    name: [metadata, name]
+              labelsFromPath:
+                exported_namespace: [metadata, namespace]
+                ready: [status, conditions, "[type=Ready]", status]
+                branch: [spec, ref, branch]
+...
+```
+
+The above configuration will result in the following metric
+
+```
+gotk_resource_info{branch="main",customresource_group="source.toolkit.fluxcd.io",customresource_kind="GitRepository",customresource_version="v1",department="baz",exported_namespace="flux-system",name="flux-monitoring",ownedBy="teamA",ready="True"} 1
+```
+
+It contains the `ownedBy="teamA"`, `department="baz"` and `branch="main"`
+labels. Similarly, more custom labels can be added depending on the need.
+
+Refer to the [kube-state-metrics custom-resource state configuration
+docs](https://github.com/kubernetes/kube-state-metrics/blob/main/docs/customresourcestate-metrics.md)
+to learn more about customizing the metrics.