Support exemplars in Metrics

[cijothomas]

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar

First stable release (1.9.0)

• SDK default behavior will be AlwaysOff. The spec says the default SHOULD be TraceBased. We decided to deviate from this due to performance concerns when Exemplars are enabled: [sdk-metrics] Turn exemplars on by default in prerelease builds #5545

• Expose APIs covered by the Exemplar diagnostic:

  This will allow users to enable Exemplars either AlwaysOn or TraceBased for all metrics.

• Add an environment variable for turning on Exemplars specifically for histograms:

  We feel Exemplars are most useful when combined with histograms. The current spec design is all-or-nothing. Due to the performance concerns, we have decided to expose an experiment mechanism for enabling Exemplars for histograms only. The plan is for this to be a temporary thing while we work to improve the spec to allow for this scenario via a stable API.

• Tasks:

  • Remove ExemplarFilter API: Exemplar - remove ExemplarFilter API #5333
  • Give ExemplarReservoir + View API its own diagnostic: Exemplar - expose ExemplarReservoir and enable View based config #5334, [sdk-metrics] ExemplarReservoir dedicated diagnostic and custom ExemplarReservoir support #5558
  • Add an experimental envvar for enabling exemplars for histograms: [sdk-metrics] Add experimental envvar for setting ExemplarFilter for histograms #5611
  • Remove experimental api decorations & update OTEL1002 diagnostic: [sdk-metrics] Expose Exemplars stable #5607
  • Add docs which show how to use the view api portion: [docs-metrics] Customer ExemplarReservoir and View API configuration #5624
  • Add example which shows how to implement a custom reservoir: [docs-metrics] Customer ExemplarReservoir and View API configuration #5624

Future release

• Propose spec changes to make it easier to enable Exemplars for specific metrics: [metrics] Allow setting ExemplarFilter via View API opentelemetry-specification#4014

• Propose spec changes to make it easier to enable Exemplars for specific instrument types: TODO

• Expose APIs covered by the ExemplarReservoir diagnostic:

  We felt the use case for building custom ExemplarReservoirs is unclear. We want to gather feedback before exposing something.

[cijothomas]

Have reviewed this in detail, and concluded the changes are non-trivial, and will not be part of 1.4.0 (Nov 2022) release. We expect to add beta support for this, shortly after 1.4.0 has shipped, with the goal of making it stable by 1.5.0 (no ETA for stable yet, but 3-4 months after 1.4.0)

[cijothomas]

Update:
1.5.0-alpha.1 added basic exemplar support. See https://github.com/open-telemetry/opentelemetry-dotnet/tree/main/docs/metrics/customizing-the-sdk#exemplars

E2E Example/Demo - https://github.com/open-telemetry/opentelemetry-dotnet/tree/main/docs/metrics/exemplars

Basic implementation only, more coming in then next few weeks.