Histogram values sum implementation

[DmitriyH]

Implementation of values total sum calculation in histograms.

The total sum of values is required to comply with Prometheus format standard:
https://prometheus.io/docs/concepts/metric_types/#histogram

Also, sum allows to calculate the average value, which could be useful for some metrics.

The corresponding issue:
#707

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[DmitriyH]

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[Anton3]

An additional atomic counter per bucket is redundant, as we only need the total sum.

But a single additional counter will cause additional contention for formats where the sum is not written, or if exact sums are not important.

Instead of a single counter, we could add a StripedCounter, but it will take extra space in case it's not needed.

As a compromise, I suggest the following:

1. Add the extra std::atomic<double> sum counter to the layout of Histogram, 1 counter per histogram.
2. Add a global std::atomic<bool> flag with a function for setting its value. If true, then sum is mutated in addition to the normal buckets.