[metrics] Allow setting ExemplarFilter via View API

[CodeBlanch]

Changes

• Allow setting ExemplarFilter via the View API

Details

In .NET we have decided to NOT enable Exemplars by default due to performance considerations. If users want Exemplars they can set an ExemplarFilter at the MeterProvider level. This will carry a performance hit for all measurements.

We feel users may only want to pay for the performance hit on critical metrics. With the current specification design, users don't have a lot of options to accomplish this. Really the only thing which may be done is enable Exemplars for the MeterProvider and then use the View API to set an ExemplarReservoir which drops Exemplars for any/all metrics where they aren't desired.

The proposed addition here is seeking to make it easier to selectively enable or disable Exemplars at the instrument-level.

For non-trivial changes, follow the change proposal process.

• Links to the prototypes (when adding or changing features): https://github.com/open-telemetry/opentelemetry-dotnet/compare/main...CodeBlanch:sdk-metrics-view-exemplarfilter?expand=1
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[jsuereth]

I don't think Views are the right solution for this level of granularity. From the specification call:

• Views are a bit of a foot-gun in that they don't compose. If you tried to enable exemplars on all histograms view a view AND any other view exists on histograms, you wind up with two metrics, not one.
• We want to limit the control of exemplars in views to ONE comprehensive single.
• We have preexisting solutions where you can configure something per-instrument via MetricReader. Adding exemplar filter here fits the same pattern we arleady have.

Would prefer to see this configuration on MetricReader, similar to temporal aggregation.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.