Ability to create metrics with different aggregators

[legendecas]

Is your feature request related to a problem? Please describe.
In the current API shape, it is not able to create a metric and set an expected aggregator. Like say, we have two observer metrics, one we'd like to aggregate with HistogramAggregator, and another with DistributionAggregator.

Describe the solution you'd like
Document the batcher API, or allow meters to accept an option of aggregation type on creating metrics.

[vmarchaud]

Do we want to put those kind of documentation inside examples or doc ?

[legendecas]

@vmarchaud example is cool but doc can still be useful.

Another point is that I checked otel impl in go, Batcher in go can determine aggregator by MetricDescriptor but not MetricKind. I'm wondering if it's ok to port this API to js before we document the batcher API, since batcher (or maybe integrator) is not been in spec yet.