Memoize Histogram Count

[vjkoskela]

Since we now compute the count of samples in a histogram on construction:

kairosdb-extensions/src/main/java/io/inscopemetrics/kairosdb/HistogramDataPointImpl.java

Line 129 in 4f54b06

this.originalCount = getSampleCount();

In order to support originalCount we should just memoize the count for purposes of sample counting as well. Of course, we'll need to handle the other case where we have a filtered histogram where the sample count differs from the original count. However, that is not the common case in my opinion.