* Graph: constant series as override
- ability to define a series override that takes the first value of a
timeseries and produces constant series from it.
* Moved constant series override logic to grafana-ui
* Added docs for instant queries and constant series override
Makes changes to dropdown-typeahead2 so that a css
class for the button can be passed in. Means it can
be used instead of dropdown-typeahead.
Switches to using dropdown-typeahead2 for series_overrides
directive and for the influxdb, mysql and postgres datasources
as it already contains a fix for this issue.
This commit also fixes the index property which
was set using an incorrectly spelled length property in the
series_overrides directive.
Closes#15621
* Karma to Jest: viewstate_srv
* Karma to Jest: annotations_srv
* Remove comments
* Karma to Jest: series_override_ctrl
* Remove unnecessary code
* Class to function and fix lint error
* Fix ngInject
