Create dashboard for service-instance-ingress consumer group lag

As a developer, I'd like to be able to detect when there are backed up event messages, so that I can see the health of the event ingestion pipeline.

We can use existing lag dashboards as a reference, e.g. https://github.com/RedHatInsights/rhsm-subscriptions/blob/5694cb04907e0c30135a76d921facc47bd522e42/.rhcicd/grafana/grafana-dashboard-subscription-watch.configmap.yaml#L1009