data: Add fill missing param to long to wide about grafana-plugin-sdk-go

[kylebrandt]

I believe it will be around "If no value found, it will have zero value":

		if currentTime.After(lastTime) { // time advance means new row in wideFrame
			wideFrameRowCounter++
			lastTime = currentTime
			for _, field := range wideFrame.Fields {
				// extend all wideFrame Field Vectors for new row. If no value found, it will have zero value
				field.Extend(1)
			}
			wideFrame.Set(0, wideFrameRowCounter, currentTime)
		}
```1

[kylebrandt]

Note: In converting to a single frame (As LongToWide does), the fillmissing in sql engine can not be fully replicated. That is because all Fields in a Frame must be of the same length. So a perfect match would require conversion to multiple Frames - []Frame.

So in a nutshell, LongToWide basically always have to have some sort of fillmode behavior baked as a natural property of the Long Schema. (Since the unique series are derived from the string field values on a long series frame, there can be "gaps" even if the type is not nullable).

[kylebrandt]

Might also consider LongToMany (but better name), and then can have a collection of series with different time index. That would be similar, but would not have to have a null value.