Decide whether to add Resource to Accumulation or keep it in the Processor

[AzfaarQureshi]

This issue captures some discussion from the November 12 2020 Python SIG.

According to the spec The Accumulator MUST provide the option to associate a Resource with the Accumulations that it produces.

Currently in the metrics SDK Resource is passed as a field to the processor because the current assumption is that Resource will remain the same across all accumulations. However, moving Resource to Accumulation could enable the Exporter to consume from multiple accumulators while keeping context of which Resource each accumulation is coming from.

The discussion in the SIG concluded that, depending on what the Python community decides, we should either create an issue in the spec to make it optional to add Resource to Accumulation, or make the required changes in our SDK.

[lzchen]

Also by definition: Accumulation: consists of Instrument, Label Set, Resource, and Aggregator snapshot, output by Accumulator and we are missing the Resource attribute as part of this class currently.

IMO, we should always adhere to the specs even if they are not set in stone, so we would:

1. Add Resource to Accumulation.
2. Remove Resource from Processor.