Logging's query importer drops all labels that are not common to both datasources.
- keep labels if label lookup of logging datasource came back empty
- easier to remove labels than having to add them in the first place
Explore is about keeping context between datasources if possible. When
changing from metrics to logging, some of the filtering can be kept to
narrow down logging streams relevant to the metrics.
- adds `importQueries` function in language providers
- query import dependent on origin datasource
- implemented prometheus-to-logging import: keeping label selectors
that are common to both datasources
- added types