Description of problem:

The HyperShift operator periodically overwrites user-defined modifications in the cluster-monitoring-config ConfigMap within a hosted control plane's openshift-monitoring namespace.

Steps to Reproduce:

1. Deploy a HyperShift Hosted Control Plane (HCP).

2. In the hosted cluster, modify the cluster-monitoring-config ConfigMap in the openshift-monitoring namespace to include custom settings. For example, apply the following configuration:

    
apiVersion: v1
kind: ConfigMap
metadata:
  name: cluster-monitoring-config
  namespace: openshift-monitoring
data:
  config.yaml: |
    alertmanagerMain:
      volumeClaimTemplate:
        spec:
          resources:
            requests:
              cpu: 90m
              memory: 1Gi
              storage: 10Gi
    prometheusK8s:
      retention: 24h
      retentionSize: 10GB
      volumeClaimTemplate:
        spec:
          resources:
            requests:
              cpu: 90m
              memory: 1Gi
              storage: 100Gi
            limits:
              cpu: 120m
              memory: 2Gi     

3. Wait for the HyperShift operator's reconciliation loop to run (observed to take approximately one hour).

4. Inspect the cluster-monitoring-config ConfigMap again: oc get cm cluster-monitoring-config -n openshift-monitoring -o yaml

Actual results:

The custom settings for alertmanagerMain and prometheusK8s are removed. The ConfigMap is overwritten and contains only the settings managed by HyperShift, losing all other configurations.

The resulting ConfigMap appears as follows:

kind: ConfigMap
apiVersion: v1
metadata:
  name: cluster-monitoring-config
  namespace: openshift-monitoring
  uid: eb93ac01-726f-4d2c-95d4-0a584bdef699
  resourceVersion: '210370596'
  creationTimestamp: '2025-08-27T14:42:56Z'
  managedFields:
    - manager: hypershift-operator-manager
      operation: Update
      apiVersion: v1
      time: '2025-08-27T15:44:06Z'
data:
  config.yaml: |
    enableUserWorkload: true
    prometheusOperator:
      logLevel: debug 

Expected results:

The HyperShift operator's reconciliation logic should be non-destructive. It should apply its required configuration while preserving any other existing, user-defined fields in the config.yaml data.The operator should merge its required configuration (e.g., enableUserWorkload: true) with any existing settings. Modifications to the ConfigMap should be additive, not a full replacement of the data block.