* Loki: Show loki datasource stats in panel inspector

- puts the loki query result stats into the query results meta stat API
of Grafana, this allows the display of all backend loki stats in the
panel inspector in the dashboards
- added a hack to also display one of those values in Explore as a meta
label using the dataframe meta `custom` mechanims to point to a single
stat entry for each series which is then added together to show total
bytes processed across all query row results (this should be changed for
7.1 to make full use of the panel inspector in Explore)

* Fix test

* nicer stats labels for loki stats with units