Clarify if MetricProducers can be registered with config instead of a method

[aabmass]

The spec defines a MetricProducer.RegisterProducer() function for binding metric producers to a reader. It says

RegisterProducer is expected to be called during initialization, but MAY be invoked later.

While implementing this in JS (open-telemetry/opentelemetry-js#4007), we already have a (stable) setMetricProducer() (singular) function which is expected to bind the reader specifically to an SDK instance. In order to implement the spec, I decided to instead add a MetricReader constructor argument accepting any additional MetricProducers.

• Is this OK spec wise? I imagine it is.
• Can/should we update the spec to make this more open ended? We have more vague wording for this kind of thing elsewhere e.g. "X SHOULD provide a way to ..."

Wanted to discuss this for #3599 @dashpole

[dashpole]

I think we should update the spec to make this more open-ended. I don't recall "late registration" being the reason we landed on the current design, so I think it is fine to relax that.