enhancements: add SEP for metrics integration

enhancements/SEP8471-metrics-integration.md

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+|Status                                             | Authors      | Created    | 
+|---------------------------------------------------|--------------|------------|
+|Implementation                                     | @mayleighnmyers, @yxun         | 2026-06-01 |
+
+# Observability Metrics Integration
+
+## Overview
+
+Upstream Istio generates telemetry metrics for the control plane and sidecar proxies. Users can customize Istio metrics with the Telemetry API and create scraping jobs by updating a Prometheus ConfigMap. For example, The following sample deploys a Prometheus instance and configures Istio telemetry integration in the Prometheus ConfigMap [samples/addons/prometheus.yaml](https://raw.githubusercontent.com/istio/istio/master/samples/addons/prometheus.yaml). However, that approach requires restarting the Prometheus instance and users may not have enough permission to update the existing Prometheus's ConfigMap.
+
+Alternatively, those can be configured by using two custom resources `ServiceMonitor` and `PodMonitor`. We want to automate the creation of `ServiceMonitor` and `PodMonitor` resources with default values in Sail Operator. And then users can query Istio metrics in Prometheus or Kiali dashboard without manual configuration step.
+
+## Goals
+
+* Provide a monitoring controller that reconciles `ServiceMonitor` and `PodMonitor` resources for Istio control plane and sidecar proxy metrics.
+* Apply platform-appropriate default relabeling rules on Kubernetes and OpenShift.
+
+## Non-goals
+
+* Deploying observability stack components (Prometheus, OpenShift Cluster Observability Operator, OpenShift User-Workload Monitoring, etc.). We assume those are installed separately.
+* Updating `ServiceMonitor` and `PodMonitor` for user customization of scraping paths, ports, or relabeling rules via the Sail Operator API.
+* Observability Distributed Tracing integration.
+* Ambient mode metrics integration is out of scope. Scraping configurations of `ZTunnel`component and Waypoint proxies are non-goals in this SEP.
+
+## Design
+
+### User Stories
+
+1. As a user running Istio with a Prometheus monitoring stack, I want the Sail Operator to configure metrics scraping jobs for Istio-generated telemetry metrics without restart the Prometheus instance.
+2. As a user running OpenShift Service Mesh, I want the Sail Operator to create `PodMonitor` resources with platform-appropriate relabeling rules. So those metrics can be formatted and displayed correctly in the OpenShift console and Kiali dashboard.
+
+### API Changes
+
+We will add a boolean field `spec.monitoring.enabled` in the `Istio` Custom Resource(CR). For example,
+
+```yaml
+apiVersion: sailoperator.io/v1
+kind: Istio
+metadata:
+  name: default
+spec:
+  monitoring:
+    enabled: false
+```
+
+This field defaults to `false`. When it is set to `true`, the monitoring controller reconciles `ServiceMonitor` and `PodMonitor` CRs for each owned `IstioRevision`. 
+
+If any custom relabeling or scraping configuration is required, users must leave `spec.monitoring.enabled` as default value `false` and they can apply independent `ServiceMonitor` and `PodMonitor` resources manually. The monitoring controller will not delete or update those independent resources in its reconciliation.
+
+#### Alternatives API Considered
+
+A broader Observability Integration API and CRD can be added in a future enhancement. It uses target reference fields and helps integrating more monitoring and tracing component services. This enhancement focuses on a monitoring controller by adding a boolean field in the `Istio` CR.
+
+### Architecture
+
+We assume `ServiceMonitor` and `PodMonitor` CRDs are available under `monitoring.coreos.com/v1` and/or `monitoring.rhobs/v1` group. The Sail Operator ClusterRole grants permissions for both API groups' `ServiceMonitor` and `PodMonitor` custom resources.
+
+A monitoring controller watches `IstioRevision` resources and reconciles `ServiceMonitor` and `PodMonitor` CRs when the boolean field `spec.monitoring.enabled` is set to `true` in the parent `Istio` CR. It also watches namespaces with the Istio sidecar injection labels and reconciles `PodMonitor` CRs for those namespaces.
+
+The monitoring controller applies platform-specific relabeling defaults when creating `ServiceMonitor` and `PodMonitor` CRs. It detects the running platform at the Sail Operator startup time.
+
+#### Monitoring Controller Design
+
+When the `spec.monitoring.enabled` is `true` in an `Istio` custom resource, the monitoring controller reconciles `ServiceMonitor` or `PodMonitor` resources and adds label such as `managed-by`: `mesh-operator` in those resources. When the `spec.monitoring.enabled` is `false` in an `Istio` custom resource, the monitoring controller stops reconciling.
+
+The monitoring controller only interacts with monitoring resources it creates. Those resources are identified by a naming convention and labels. It will not delete or update user-managed `ServiceMonitor` or `PodMonitor` resources that were configured manually in existing environments.
+
+The controller creates and reconciles `ServiceMonitor` resources named as `{IstioRevision.name}-istiod-metrics` in the control plane namespace. And it creates and reconciles `PodMonitor` resources named as `{IstioRevision.name}-proxies-metrics` in each matching application namespace. It uses Get/Create/Update methods and does not list or bulk-delete monitoring resources in a namespace.
+
+#### Custom Resource Deletion
+
+The **ServiceMonitor** custom resource carries an `ownerReferences` field which points at the owning `IstioRevision` resource. When the `IstioRevision` resource is deleted, Kubernetes garbage collection removes the `ServiceMonitor` resource. 
+
+The **PodMonitor** custom resource does not carry an `ownerReferences` field because cross-namespace owner references are not supported. Those resources are not automatically garbage-collected when the owner `IstioRevision` is deleted. The monitoring controller explicitly handles the cleanup of those `PodMonitor` resources in the following conditions:
+- The owner `IstioRevision` is deleted. 
+- The sidecar injection label(s) are dropped or disabled in application namespaces.
+
+#### Sidecar injection namespace selection
+
+The monitoring controller reconciliation watches namespaces with the Istio sidecar injection labels. For example:
+
+| Namespace label | Resolved revision |
+|-----------------|-------------------|
+| `istio-injection: enabled` | `default` |
+| `istio.io/rev: <name>` | `<name>` |
+
+#### Kubernetes vs OpenShift
+
+On **Kubernetes** platform, the monitoring controller creates a `PodMonitor` resource in the control plane namespace. The resource uses a `namespaceSelector` field for namespace selection.
+
+On **OpenShift** platform, the default Prometheus monitoring stack ignores a `namespaceSelector` field, so we need create `PodMonitor` resources in those sidecar injection enabled namespaces.
+
+#### External CRD/API Group Detection
+
+The monitoring controller should detect which Operator API group is available before creating `ServiceMonitor` or `PodMonitor` resources:
+
+| API group | Typical platform |
+|-----------|------------------|
+| `monitoring.coreos.com/v1` | Kubernetes Prometheus Operator |
+| `monitoring.rhobs/v1` | OpenShift Cluster Observability Operator |
+
+By default, the monitoring controller will create the `ServiceMonitor` and `PodMonitor` resources using the `monitoring.coreos.com` group.
+
+#### Monitoring Availability
+
+When the `spec.monitoring.enabled` is `true` in an `Istio` custom resource, but the required `ServiceMonitor` and `PodMonitor` CRDs are not installed, the monitoring controller should set a status condition in the `Istio` custom resource. For example:
+
+| Field | Value |
+|-------|-------|
+| `type` | `MonitoringAvailable` |
+| `status` | `False` |
+| `reason` | `MissingCRDs` |
+| `message` | `ServiceMonitor and PodMonitor CRDs not found` |
+
+### Performance Impact
+
+The `ServiceMonitor` reconciliation is only required in a mesh's control plane namespace and it has low performance impact. 
+The `PodMonitor` reconciliation watches namespaces with the sidecar injection labels on OpenShift platform. It may impact performance when there are large number of namespaces in a cluster.
+
+## Implementation Plan
+
+- [x] Monitoring controller reconciling `ServiceMonitor` and `PodMonitor` for `IstioRevision`
+- [x] `Istio.spec.monitoring.enabled` API
+- [x] Platform-specific PodMonitor relabeling defaults
+- [x] Unit tests for controller and relabeling package
+- [ ] External CRD/API group (`monitoring.coreos.com` and `monitoring.rhobs`) detection 
+- [ ] Kubernetes PodMonitor strategy using `namespaceSelector.any: true`
+- [ ] Set a status condition in the `Istio` custom resource
+- [ ] Add Integration tests such as `tests/integration/api/monitoring_test.go`
+- [ ] Add KinD e2e tests
+- [ ] User-facing documentation
+
+Alternatives API Considered
+- [ ] Introduce an Integration API in v1alph1 api group. It uses target references fields for integrating more Observability services.
+
+## Test Plan
+
+Functionality will be tested in unit tests, integration tests (envtest), and KinD e2e tests.
+
+## Updates
+
+* 2026-06-11: Updated with platform relabeling implementation, remaining v1alpha1 work items.
+* 2026-06-23: Updated Overview, Goals, Non-goals, Design descriptions. Added Alternatives API Considered. Removed OSSM JIRA references.
+* 2026-07-01: Update descriptions. Addressed review comments. Renamed this file with JIRA work item number.