HistogramValue.Bucket.Exemplar is inadequate

[jmacd]

The HistogramValue Bucket supports one exemplar per bucket, with exemplar values being restricted to string-valued attributes. This is particularly restrictive, for a number of reasons:

1. It's not possible to provide exemplars for non-histograms
2. It's not possible to provide more than one exemplar per bucket
3. It requires converting certain data types to a string, which will require additional specification

I would remove this support entirely until a full set of requirements can be drafted. (1) It's meaningful and reasonable to support encoding exemplars for any kind of metric instrument. (2) It's certainly useful to provide more than one exemplar per instrument/value-range, as a means of conveying a sample distribution, (3) There is a common request (e.g., open-telemetry/opentelemetry-specification#381, https://kccncna19.sched.com/event/UaXX/deep-linking-metrics-and-traces-with-opentelemetry-openmetrics-and-m3-rob-skillington-chronosphere) to include trace context in these exemplars, and forcing them to be encoded as string-attribute lists is onerous (and inefficient)--probably trace context should be specialized.