Add support for configuring exemplar filter

[jack-berg]

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

Need declarative config equivalent of OTEL_METRICS_EXEMPLAR_FILTER, which supports values always_on, always_off, trace_based.