Added thread option and enabled monit for Prometheus

Reference to enable monit for Prometheus:
https://book.kubebuilder.io/reference/metrics.html

The usage of Kubebuilder as reference comes from the deprecated implementation on the OperatorSDK:
https://sdk.operatorframework.io/docs/building-operators/golang/advanced-topics/#metrics

Added new var to Operator, parallel-thread-count.

Partially solves https://gitlab.cern.ch/webservices/webframeworks-planning/-/issues/575