* Add rate interval variable to prometheus data source
* Add tests + auto complete
* Fix prometheus tests
* Add doc
* Modify test title
* Modify kbn method name after merge
- `unless`, `or` and `and` are binary operators
- if they appear in a query the query was broken up so the suggestor
only works on the current term
- this change fixes the splitter logic to make sure those keywords are
not inside a word
* 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
* Prometheus: disable dynamic label lookup on big datasources
- when a prometheus datasource has more than 10000 metrics, label lookup
for the query field is disabled
- installations of that size have slow typehead lookup times and make
the editor sluggish
* Raise dynamic lookup threshold to 10000 metrics
* Run start tasks again
