Loki/Prometheus: updates addLabelToQuery method to prevent it from adding labels outside of selector when using Loki datasource (#25059)

* Chore: updates add label to query method in prometheus datasource to fix loki label insert

* Chore: adds addLabelToQuery test covering differences between adding label to loki vs non-loki queries

* Chore: adds an additional comment to addLabelToQuery

* Chore: renames isLokiDatasource to hasNoMetrics in addLabelToQuery