this is a fix-up PR that cleans up Explore Logging after the recent
restructuring.
- log results need to be merged since query transactions have been
introduced
- logging DS has its own language provider, query field, and start page
(some of them based on prometheus components)
- added loader animation to log viewer
- removed logging logic from prometheus components
- when instantiating a datasource, the datasource service checks if the
plugin module exports Explore components, and if so, attaches them to
the datasource
- Explore component makes all major internal pluggable from a datasource
`exploreComponents` property
- Moved Prometheus query field to promehteus datasource and registered
it as an exported Explore component
- Added new Start page for Explore, also exported from the datasource
