Histogram field type support for ValueCount and Avg aggregations

[csoulios]

Implements value_count and avg aggregations over Histogram fields as discussed in #53285

• value_count returns the sum of all counts array of the histograms
• avg computes a weighted average of the values array of the histogram by multiplying each value with its associated element in the counts array

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)

[nik9000]

I left some small stuff but it mostly looks good to me. Its neat that this is comparatively easy to do now!

[nik9000]

@not-napoleon and I've been favoring changing the ctor so you can just to (MetricAggregatorSupplier) HistoBackedSumAggregator::new. It is less "big".

[csoulios]

(MetricAggregatorSupplier) HistoBackedSumAggregator::new is definitely better looking and I would prefer it as well.

The problem is that MetricAggregatorSupplier passes a ValuesSource argument

Aggregator build(String name,
                     ValuesSource valuesSource,
                     DocValueFormat format,
                     SearchContext context,
                     Aggregator parent,
                     Map<String, Object> metadata) throws IOException;

while HistoBackedSumAggregator constructor expects a HistogramValuesSource.Histogram

HistoBackedSumAggregator(String name, HistogramValuesSource.Histogram valuesSource, DocValueFormat formatter, SearchContext context,
            Aggregator parent, Map<String, Object> metadata) throws IOException 

So I had 3 options:

1. Keep the ValuesSource parameter in the HistoBackedSumAggregator constructor and do a type cast later when I would use the HistogramValuesSource.Histogram object.
2. Create a separate HistoBackedMetricAggregatorSupplier that would match the arguments of the HistoBackedSumAggregator
3. Do the cast of ValuesSource to HistogramValuesSource.Histogram in the registrar method changing the admittedly prettier (MetricAggregatorSupplier) HistoBackedSumAggregator::new to the longer lambda expression.

I chose the third option because it seemed cleaner to do the type cast at the registrar method where it is obvious that the VS is a histogram. On the other hand, I didn't want to create yet another AggregatorSupplier sub-class

[not-napoleon]

I've been favoring having generic ValuesSource in the supplier and doing the cast in the aggregator constructor instead. Really the suppliers shouldn't care about the ValuesSource subclass, since the whole idea is to be able to add new ones in, and having to subclass a supplier to do that is a lot of boiler plate. I know I haven't been perfectly consistent about that so far, but I'm working on cleaning it up as I can.