Monitoring of Jaeger instance

The way to tell Prometheus to monitor an endpoint in Kubernetes is by creating a ServiceMonitor k8s object. We already do that for other services deployed in k8s. ServiceMonitor is a CRD introduced by Prometheus.

We use jaeger-operator for managing Jaeger and it's not uncommon for operators to support the creation of ServiceMonitor objects. So we'll probably want to use that.

The purpose of this issue is to investigate if adding ServiceMonitor is supported by the jaeger-operator, extend the jaeger.libsonnet library in tanka-deployments and enable it in different envs.