Fix and optimize getValuesAtIntervalOp data extraction.

- only the data extracted in the last loop iteration of ExtractSamples() was
  emitted as output
- if e.g. op interval < sample interval, there were situations where the same
  sample was added multiple times to the output
Merge pull request #246 from prometheus/fix/interval-value-extraction

Fix and optimize getValuesAtIntervalOp data extraction.