Part of https://github.com/open-telemetry/opentelemetry-go/issues/5249
This makes all existing types designed to implement the public Exemplar
API public by moving most of `internal/exemplar` to `exemplar`. The only
types that are not being made public are `exemplar.Drop`, and
`exemplar.FilteredReservoir`. Those types are moved to
`internal/aggregate`, and are renamed to `DropReservoir` and
`FilteredExemplarReservoir`.
The following types are made public:
* `exemplar.Exemplar`
* `exemplar.Filter`
* `exemplar.SampledFilter`
* `exemplar.AlwaysOnFilter`
* `exemplar.HistogramReservoir`
* `exemplar.FixedSizeReservoir`
* `exemplar.Reservoir`
* `exemplar.Value`
* `exemplar.ValueType`
* Refactor exemplars to not use generic argument
* Update internal/aggregate
* Update metric SDK
* Test exemplar value type
* Add TestCollectExemplars
* Fix lint
---------
Co-authored-by: Sam Xie <sam@samxie.me>
