Adjust return signature of GetBoundaryValues() metric persistence method

[juliusv]

Current:

GetBoundaryValues(_model.LabelSet, *model.Interval, *StalenessPolicy) (_model.Sample, *model.Sample, error)

The return value is hard to use because the caller needs to manually match labelsets between the first and second return value e.g. for computing deltas. And there is nothing in the return types itself that ensures that the labels even do match.

So I think it should be the same as the GetRangeValues() return value:

GetBoundaryValues(_model.LabelSet, *model.Interval, *StalenessPolicy) (_model.SampleSet, error)

Downside: it's not explicit from the return type that it contains exactly two datapoints in each timeseries, but it's probably better than introducing yet another special type that ensures that.

[matttproud]

@juliusv, this is likely obsolete.

[lock]

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