Default aggregation should mention "Attributes" from instrument advisory

[cijothomas]

What are you trying to achieve?

The default aggregation is only referring to using the instrument advisory parameter for ExplicitBucketBoundaries, but it should also include the instrument advisory parameter for Attributes.

What did you expect to see?

In the absence of view, SDKs are expected to use the instrument advisory parameter "Attributes", and use them for aggregation. This information is currently mentioned, but somewhat hidden under Views section on "Stream configuration". But it should be specified in the default aggregation section.

Additional context.

Given the experimental nature of "Attributes", this might be non-trivial to add without some refactoring of the aggregation section.