Expose the size of batch metrics histograms, to be aware of the memory consumption for pipeline.

[andsel]

Every pipeline uses various instances of histograms data structure to provide the batch's structural metrics ((byte size and event count) on the various time windows.

Compute the memory consumption of such histograms per pipeline or globally, so that there a clear idea of what to expect in terms of memory usage.
This could be logged and maybe also exposed as metric.