Avoid reporting zero sample counts for sparse metrics

Fixes bug 1077042

Previously for sparse metrics (i.e. where there exist only relatively
few actual datapoints), the responses to calls on the GetMetricStatistics
API seeking the SampleCount statistic were polluted with many trivial
datapoints for the periods in which there were no metrics reported.

Now these meaningless & unnecessary data are excised from the response,
as they may be inferred.