Histogram Reservoir should use a time-unbiased sampling algorithm

[dashpole]

Problem Statement

While working on #8230, I realized that open-telemetry/opentelemetry-specification#4678 wasn't ever implemented.

Using a time-unbiased algorithm is more performant because we would sample observations, and storing the exemplar is the expensive part of Offer.

It also provides more useful data, since exemplars are spread out across time.

Proposed Solution

Re-use parts of the fixed-size reservoir to make the histogram reservoir time-unbiased.

Alternatives

A clear and concise description of any alternative solutions or features you've considered.

Prior Art

Previous PoC: #7458

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