[eck-operator] No possibility to configure custom SM labels, interval and etc

[kseniyashaydurova]

Proposal

Pls add possibility to configure custom ServiceMonitor parameters for eck-operator. Current HELM chart provides possibility to define prometheus ServiceMonitor params - https://github.com/elastic/cloud-on-k8s/blob/main/deploy/eck-operator/values.yaml#L323 But in reality it allows to configure only its namespace and nothing else - https://github.com/elastic/cloud-on-k8s/blob/main/deploy/eck-operator/templates/serviceMonitor.yaml#L6

Use case. Why is this important?

Currently we can't configure SM interval and what is more important to add custom labels and annotations. Adding custom labels is useful on eck-operator + prometheus integration, cause it needs a release label to be passed. Otherwise prometheus won't add SM as target.