Description of problem:

ACM registrastion removes `retentionSize` from Cluster monitoring operator configmap

Version-Release number of selected component (if applicable):

CM 2.7.7 observability and openshift 4.12 

How reproducible:

Always

Steps to Reproduce:

1. How to reproduce:
   1) Edit the configmap cluster-monitoring-config in namespace openshift-monitoring to add a retentionSize config
   2) “register“ the modified cluster to ACM with the observability addon (or wait ~4days according to the logs). The endpoint-observability-operator located in namespace open-cluster-management-addon-observability on the targeted cluster will update the cluster-monitoring configmap to add the ACM alertmanager url and remove the retentionSize field.

Actual results: 

retentionSize field removed

Expected results:

retentionSize field remains

Additional info: