metrics: add H2 Histogram option to improve histogram granularity

[rcoh]

Motivation

Closes: #6896

Solution

This PR introduces a configurable new histogram for the poll-time histogram based on H2 Histograms. As part of this, three APIs are deprecated:

• metrics_poll_count_histogram_scale
• metrics_poll_count_histogram_resolution
• metrics_poll_count_histogram_buckets

Since I suspect these are commonly used, I thought it was worth a little added complexity to provide a graceful path to removing them.

They are replaced with metrics_poll_count_histogram_configuration that accepts a histogram configuration object. The intention of this change is to provide more knobs when configuring histograms (as customers will want with the H2 histogram).

The default histogram remains the linear histogram with 10 buckets. The default H2 histogram provides a 25% error bound from 100ns to 60s of poll time and uses about 1KB of storage. This is configurable by customers as well.

The h2_histogram code is fairly complex compared to the code for the other histogram modes, so I isolated it into its own module. I brought back proptest to test invariants across a wide range of possible parameters–since there isn't any actual branching in this code, we don't get any value from making them fuzz tests (other than that they are a little more annoying to run).

[carllerche]

I'm fine w/ this. Thanks.

[hds]

I have a couple of suggestions on code style and some formatting nits, but this look good to me.

There is one place I mentioned where I think some more description on what the examples are doing would help people understand the configuration options.

[hds]

Couple more small things.