Stochastic test support plural SampleValueSeries.

After SampleValue was refactored into SampleValueSeries, which
involves plural values under a common super key, the stochastic
test was never refreshed to reflect this reality.  We had other
tests that validated the functionality, but this one was
insufficently forward-ported.
Refresh SampleValue to 64-bit floating point.

We always knew that this needed to be fixed.
Merge pull request #164 from prometheus/extensibility/futureproof/values

Refresh SampleValue to 64-bit floating point.