Loki metadata available at variable's querying

**Why is this needed**: Loki metadata provides more flexibility to design query without worrying about cardinality problems. To fully enable loki's powers to create dashboards with Grafana the metadata querying should be also available at variables. **What would you like to be added**: I'm currently migrating our stack from elastic to grafana. Some of the filters available on elastic's dashboard can't be used at Loki due to the lack of metadata variable's querying feature. **Who is this feature for?** All Grafana users that uses the newest versions of Loki as data source