Explore: query transactions for faster result display

* stackdriver: add resource type to filter and group bys

* stackdriver: remove not used param

* stackdriver: refactor filter and group by code

* stackdriver: remove resource type if its already in filter list

* stackdriver: remove debug logging

* stackdriver: remove more debug logging

* stackdriver: append resource type to legend name if there are more than one type present in the response

* stackdriver: only make new request if filter has real value

* stackdriver: format legend support for resource type

* stackdriver: add resource type to documentation

* stackdriver: not returning promise from query function

* stackdriver: fix refactoring bug

* stackdriver: remove not used import

[skip ci]

Another fix for kiosk mode handling

Kiosk mode url fix

Existing querying was grouped together before handed over to the
datasource. This slowed down result display to however long the slowest
query took.

- create one query transaction per result viewer (graph, table, etc.)
  and query row
- track latencies for each transaction
- show results as soon as they are being received
- loading indicator on graph and query button to indicate that queries
  are still running and that results are incomplete
- properly discard transactions when removing or changing queries

Feat - keyboard shortcut to toggle legend

Explore: reuse table merge from table panel

ux: remove duplicate placeholder attribute

[skip ci]

[skip ci]

* fix: text box template variable doesn't work properly without a default value

Add debug logging of dashboard permission checks

Stackdriver: Add possibility to authenticate using GCE metadata server