Expose ExemplarReservoir as configuration option of View.

[jsuereth]

Fixes #2922
Fixes #3812
Related #3756

Changes

• Cleans up language around exposing ExemplarReservoir. (Remove TODO, e.g.)
• Remove ExemplarFilter interface but keep configuration options. (see Can the exemplar filter and reservoir be merged? #3812)
• Clarify / simplify Spec Compliance matrix for existing state of the Exemplar Specification.

Prototype in java: open-telemetry/opentelemetry-java#5960

[cijothomas]

How is #2922 resolved by this PR? I am not able to see how that issue is being addressed here.

[MrAlias]

How is #2922 resolved by this PR? I am not able to see how that issue is being addressed here.

A user will be able to implement their own exemplar reservoir that accomplishes the behavior desired in #2922. With this change the metric pipeline will be able to accept and use that reservoir.

[cijothomas]

Remove ExemplarFilter interface but keep configuration options.

Can you add this to changelog as well ? Also, its not clear from PR description why we are removing the ability to author custom ExemplarFilter? Is that temporarily done to unblock stable release? Or we don't see the need of custom ExemplarFilters at all?

[cijothomas]

How is #2922 resolved by this PR? I am not able to see how that issue is being addressed here.

A user will be able to implement their own exemplar reservoir that accomplishes the behavior desired in #2922. With this change the metric pipeline will be able to accept and use that reservoir.

I see. It'd good to describe in PR desc, how is this achieved so future readers can know how to achieve. Otherwise, merging this PR would just close the associated issue, without a clear description of how it was closed.

[jsuereth]

Remove ExemplarFilter interface but keep configuration options.

Can you add this to changelog as well ? Also, its not clear from PR description why we are removing the ability to author custom ExemplarFilter? Is that temporarily done to unblock stable release? Or we don't see the need of custom ExemplarFilters at all?

Yeah, there was concern in the Specification meeting that ExemplarFilter duplicates work w/ ExemplarReservoir and users may be confused when to use one vs. the other. Rationale behind the decision is in: #3812.

We're err-ing on "less in the initial release is better", and we have the ability to bring back the interface if needed in the future.

[carlosalberto]

@jsuereth I think we are more than ready to merge this one! Please resolve the conflicts in the compliance matrix, etc.