Omnibus Praefect dashboard improvements from demo

From the 2020-04-14 demo, we noticed the following potential improvements:

• The table containing the latest known elected primary should also include the instance name (e.g. IP or hostname).
• The replication queue size could be further broken down by Praefect instance and virtual storage.
• Need a panel for the replication delay metric (gitaly_praefect_replication_delay).
• Virtual storage flapping should use two labels ({{instance}} {{gitaly_storage}})

From the 2020-04-17 demo, we noticed we should also include:

• A panel for the query rate(grpc_server_handled_total{job="praefect",grpc_code!="OK"}[5m])
• A panel for the node up time: gitaly_praefect_node_last_healthcheck_up