Adding empty label selector for prom service monitor selector

[ezio-auditore]

Introduced in #2465 the service monitor selector in the Prometheus configuration was marked wrongly,hence it was scraping metrics from only k8s-service-monitor. Adding an empty service monitor selector scrapes metrics from all Service monitors present in the namespace.
List of Service Monitors present in the namespace

k8s-metrics-service-monitor                      
noobaa-mgmt-service-monitor                    
ocs-metrics-exporter                            
odf-operator-controller-manager-metrics-monitor  
rook-ceph-exporter                              
rook-ceph-mgr                                    
s3-service-monitor                                

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ezio-auditore, umangachapagain

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [umangachapagain]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ezio-auditore]

/cherry-pick release-4.16

[openshift-cherrypick-robot]

@ezio-auditore: new pull request created: #2637

In response to this:

/cherry-pick release-4.16

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.