Loosen "MetricProducer MUST accept configuration for the Resource" requirement

[aabmass]

Initially discussed in #3601 (comment). The spec currently says

opentelemetry-specification/specification/metrics/sdk.md

Lines 1398 to 1400 in 8ae0f6f

	If the batch of [Metric points](./data-model.md#metric-points) returned by
	`Produce()` includes a [Resource](../resource/sdk.md), the `MetricProducer` MUST
	accept configuration for the [Resource](../resource/sdk.md).

Since SDK's may throw away the Resource returned by a MetricProducer, we can loosen this requirement to a SHOULD or MAY

Part of #3599 @dashpole

[aabmass]

I can send a PR for this if someone assigns me