* Prometheus: Display HELP and TYPE of metrics if available
- Prometheus recently added a metadata API around HELP and TYPE of
metrics
- request metadata when datasource instance is created
- use metadata to show help and type in typeahead suggestions and in
metrics selector as tooltip
* Fix types
- 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
