* Make rate possible non-counter annotation consistent
Previously a PossibleNonCounterInfo annotation would be left in cases
where a range-vector selects 1 float data point, even if no more points
are selected in order to calculate a rate.
This change ensures an output float exists before emitting such an
annotation.
This fixes an inconsistency where a series with mixed data (ie, a float
and a native histogram) would emit an annotation without any points.
For example,
```
load 1m
series{label="a"} 1 {{schema:1 sum:10 count:5 buckets:[1 2 3]}}
eval instant at 1m rate(series[1m1s])
```
Would have a PossibleNonCounterInfo annotation.
Wheras
```
load 1m
series{label="a"} {{schema:1 sum:10 count:5 buckets:[1 2 3]}} {{schema:1 sum:15 count:10 buckets:[1 2 3]}}
eval instant at 1m rate(series[1m1s])
```
Would not.
---------
Signed-off-by: Joshua Hesketh <josh@nitrotech.org>
b6107cc888