Tracking: Exemplar Specification - Stabilization

[jsuereth]

This bug is tracking issues that must be resolved before Exemplars can be declared stable.

• Exemplar Reservoir Offer should be allowed to only accept dropped attributes #3669
• Update default exemplar reservoir size for base-2 exponential histogram buckets #3674
• Exemplar clarifications #2421
• Incomplete example implementation of naive reservoir sampling algorithm #2205
• Can the exemplar filter and reservoir be merged? #3812
• Exposing ExemplarReservoir for users to override its behavior. (This unblocks feature requests like Attribute to indicate whether a Span is used as an Exemplar #2922)
• Revert Remove the "MUST" wording which requires exemplar to be on-by-default #2414 or otherwise enable exemplars by default w/ SHOULD/MAY requirement.
• Clarify that exemplar reservoir default may change in a minor version #3943

[jack-berg]

Added Clarify that exemplars are best effort and changing defaults are non-breaking per this conversation: #3870 (comment)

[reyang]

The TC discussed this during the Mar. 13th, 2024 meeting:

1. There is only one blocking issue (more details can be found from Tracking: Exemplar Specification - Stabilization #3756), @jack-berg is working on it Clarify that exemplar reservoir default may change in a minor version #3943.
2. After PR Clarify that exemplar reservoir default may change in a minor version #3943 is merged, we should be ready to move this PR forward. We're targeting mid Apr. for the Metrics Exemplar spec to be marked Stable (spec release will happen early May).