Exemplar support

[jmacd]

Is your feature request related to a problem? Please describe.
The OTel metrics SDK specification has an experimental section on Exemplars.
The group is moving to stabilize that spec, and this reference implementation ought to have an implementation.

Describe the solution you'd like
Because we have a two-stage aggregation pipeline, the "Simple" reservoir approach is not possible. We use a weighted reservoir instead, because it correctly aggregates inputs and preserves their weight.