When there is a single frame with no fields (e.g. splunk datasource) SSE errors when trying to figure out the data type. This frame needs to exist since this is where the executedQueryString metadata exists.
This adds a new return type to SSE to represent no data, so the original frame with its metadata can still be maintained.
Instead of using the frame name as the series name, we now use the name of the series value column if the frame name is not set. Queries to some datasources may end up with an empty frame name, which leads to an empty series name. As the series name is used in the frontend to display which metrics were queried etc. it shouldn't be empty.
* SSE: fix reduce to handle non-null
* add type for data.Field that is float64 or *float64
* resample fix for non-null input case, add couple non-null tests
- Add the ability to stick metadata attached to a value in expressions. Currently uses Frame.Meta.Custom.
- None of this is consumed by anything yet, so an incremental step.
* comes from grafana/gel-app
* remove transform plugin code
* move __expr__ and -100 constants to expr pkg
* set OrgID on request plugin context
* use gtime for resample duration
* in resample, rename "rule" to "window", use gtime for duration, parse duration before exec
* remove gel entry from plugins-bundled/external.json
which creates an empty array for plugins
