Clarify on the term Aggregation

[legendecas]

What are you trying to achieve?

In the metrics SDK spec, the term Aggregation is declared to "informs the SDK on the ways and means to compute Aggregated Metrics from incoming Instrument Measurements."

Also, there is a pre-defined aggregation DefaultAggregation which acts like the previously defined AggregatorSelector, and SumAggregation and HistogramAggregation etc. that act like previously defined concrete type SumAggregator and HistogramAggregator. With #2153 the term Aggregator will be reserved in the spec, it is still unclear what the Aggregation is capable of: just selecting the aggregation means, or does it also handle the concrete aggregation jobs?

Since views are configured with aggregations, I found that it is confusing on the term Aggregation as it is defined like the selector role but used the term for concrete means to compute aggregated metrics.

[legendecas]

Continuing the discussion on #2149