Results.html: record maximum Histogram.numValues in google analytics.
This CL records an instant event 'HistogramParameterCollector.maxSampleCount'
containing the maximum numValues from the raw Histograms that were produced directly
by metrics, before the Histograms are merged.
We know that this number is always 1 for some metrics.
If this number is always 1 across *all* metrics, then we may want to simplify the
metrics API from Histograms to Scalars. In that case, Histograms would still be
necessary as an implementation detail for results.html and the dashboard Histogram
Pipeline, but the metrics API could be simplified.
If this number is not always 1, then it will be easier to explain why the metrics
API requires Histograms instead of the seemingly simpler Scalars.