- adds a custom label renderer to Logs viewer in Explore
- labels are no longer treated as strings, they are passed as parsed objects to the log row
- label renderer supports onClick handler for an action
- renamed Explore's `onClickTableCell` to `onClickLabel` and wired up log label renderers
- reuse Prometheus `addLabelToSelector` to modify Logging queries via click on label
- added tests to `addLabelToSelector`, changed to include the surrounding `{}`
- use label render also for common labels in the controls panel
- logging meta data section has now a custom renderer that can render numbers, strings, and labels
- style adjustments
