* fix(explore): Prevent double querying for Prometheus and Loki

- queries were triggered twice because two Enter handlers existed
- removed runner plugin from Loki and Prom query fields (runner plugin
is still being used in azure)

Part of #16995

* Set loki's mtrics capability to false

Loki is not a metrics store. Explore was using the `metrics` field in
the plugin definition to issue a second query run. But Loki shows only
one result viewer.

Fixes #16995