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Part of https://github.com/open-telemetry/opentelemetry-go/issues/5249 ### Spec https://opentelemetry.io/docs/specs/otel/metrics/sdk/#exemplarfilter > The ExemplarFilter configuration MUST allow users to select between one of the built-in ExemplarFilters. While ExemplarFilter determines which measurements are eligible for becoming an Exemplar, the ExemplarReservoir makes the final decision if a measurement becomes an exemplar and is stored. > The ExemplarFilter SHOULD be a configuration parameter of a MeterProvider for an SDK. The default value SHOULD be TraceBased. The filter configuration SHOULD follow the [environment variable specification](https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/#exemplar). > An OpenTelemetry SDK MUST support the following filters: > * [AlwaysOn](https://opentelemetry.io/docs/specs/otel/metrics/sdk/#alwayson) > * [AlwaysOff](https://opentelemetry.io/docs/specs/otel/metrics/sdk/#alwaysoff) > * [TraceBased](https://opentelemetry.io/docs/specs/otel/metrics/sdk/#tracebased) ### Changes * adds exemplar.AlwaysOffFilter, which is one of the required filters from the SDK: https://opentelemetry.io/docs/specs/otel/metrics/sdk/#alwaysoff * adds `metric.WithExemplarFilter` as an option for the metrics SDK. * moves handling of `OTEL_METRICS_EXEMPLAR_FILTER` to the same location as config handling to make code easier to navigate. dropReservoir can actually be removed, but I plan to do that in a follow-up refactor, since it will be a large diff. --------- Co-authored-by: Damien Mathieu <42@dmathieu.com> Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com> Co-authored-by: Chester Cheung <cheung.zhy.csu@gmail.com>