[Improve] The default variables query of panel pulsar-topics will trigger full query

[Gleiphir2769]

In the panel pulsar-topics, the query for cluster/namespace/topic may hang prometheus.

apache-pulsar-grafana-dashboard/dashboards/topic.json

Line 1095 in abf90c1

"query": "{namespace=~\".+\"}",

apache-pulsar-grafana-dashboard/dashboards/topic.json

Line 1115 in abf90c1

"query": "{topic=~\".+\"}",

apache-pulsar-grafana-dashboard/dashboards/topic.json

Line 1075 in abf90c1

"query": "{cluster=~\".+\"}",

It will query all the metrics to find all the cluster/namespace/topics, which may cause prometheus crash.