Hi, I'm working on adding exemplar support for histograms to a Haskell 
prometheus client. I wanted to run my implementation by Prometheus folks to 
see if this all sounds correct:

1. For a given histogram with 5 buckets, store up to 1 exemplar per bucket. 
Don't store exemplars for +Inf samples.
2. There should be a minimum retention period, such that if you store two 
exemplars for the same bucket back-to-back, only the first exemplar is 
stored. My understanding is there are two main purposes of this:

a. Store fewer exemplars in prometheus
b. Letting callers know up-front if an exemplar is likely to make it to 
prometheus, so that they can sample the trace.

3. There should be a maximum retention period, such that an exemplar is no 
longer sent to prometheus if it's e.g. older than a minute. I'm less clear 
on why this is desired, but want to give a good explanation in docs.

WIP PR here: https://github.com/fimad/prometheus-haskell/pull/74

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