Add exemplar reservoir support to View

[tylerbenson]

Currently there are options defined in the spec that are not available to configure (for example exemplar_reservoir).

I would like the ability to configure the reservoir size and even exemplar filter per metric. Neither of those are possible with the current API.
(SdkMeterProviderUtil.setExemplarFilter allows configuring the filter for the whole meterProvider, but not per metric. It should probably be renamed to setDefaultExemplarFilter and allow views to define alternate filters.)

[jack-berg]

Hey we chatted on slack about this a bit but just wanted to repeat a few things for context of other readers now that there's an issue.

Exemplars are stable at the spec, but not yet stable in opentelemetry-java. We can / should push to stabilize them, but there's a few things that stand in the way:

• ExemplarFilter is no longer an SDK extension plugin interface. The java implementation needs to be adjusted to somehow add guardrails that allow a user to select the built in implementations (tracebased, alwaysOn, alwaysOff) without allowing programmable customization.
• Review the exemplar API and decide which elements should be made public.
  • ExemplarReservoir is a SDK extension plugin interface, which matches the java implementation.
  • Consider refactoring the way you access built-in ExemplarReservoir implementations to match our other API design. I.e. dedicated create methods in each respective implementation class.
  • Address other API design issues that will inevitably come up along the way.

You mention that we should rename SdkMeterProviderUtil.setExemplarFilter to setDefaultExemplarFilter to reflect that the value can be overridden at the view level, but my understanding is that only exemplar reservoir is configurable at the view level, not exemplar filter.

Additionally, as you point out, the spec now states that exemplar reservoir is configurable at the view stream level, which is not implemented in java.

It would be easiest to add this once exemplars are already stable.

We can of course use our toolkit of internal classes and reflection to add support while exemplars are still experimental. This would entail:

• Adding package private setter for exemplar reservoir on ViewBuilder
• Adding SdkMeterProviderUtil.setExemplarReservoir(ViewBuilder, ExemplarReservoir), which would be responsible for reflectively calling the new ViewBuilder package private exemplar reservoir setter

The downside of adding experimental APIs for this purpose is that they will churn in the near future when exemplars stabilize.