Context:

With OPT-4212, we created the NumberVariableAggregationService wich includes the shard groupBy and distributedBy number variable aggregation creation logic, which is currently done using a range aggregation. 

Lets refactor this to use a histogramAggregation, which will enable us to use the logic currently contained in DurationAggregationService.
We might need to add a more generalised service to handle the creation of (non date) histogram aggregations with a sub aggregation (the groupByVariable will need a histogram with distributedBy subaggregation, the distributedByVariable will need a histogram with viewPart subaggregation) which can then be used to create duration or variable histograms.

Note:
Double variable histrogram bucket keys should have two decimal places. If we add a format to the histogram ("0.00" format) for double variable bucket keys this should resolve OPT-4238, since the bucket keys will be rounded before evaluation.

AT:

• number variable aggregations are handled with histograms
• Duration & NumberVariableAggregationServices are merged where possible
• IT is added to ensure this has resolved OPT-4238