Add support for specifying exemplar reservoir

[Exemplar reservoir](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplarreservoir) is an SDK extension plugin interface which is a configuration parameter of a [view stream](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#stream-configuration). 