Fix aggregation.Default to properly return the default one

[dmathieu]

About aggregation.Default{}, the spec says:

The Default Aggregation informs the SDK to use the Instrument Kind (e.g. at View registration OR at first seen measurement) to select an aggregation and configuration parameters.

However, right now, if we use that default in an aggregation selector, we get the following error:

could not create all aggregators: creating aggregator with instrumentKind: 2, aggregation {}: unrecognized aggregation: {}

This fixes that issue, ensuring the default aggregator is properly found, as per the spec (and the aggregation.Default godoc).

[codecov]

Codecov Report

Merging #3967 (97d6084) into main (6b7e207) will increase coverage by 0.0%.
The diff coverage is 100.0%.

❗ Current head 97d6084 differs from pull request most recent head f58dd3a. Consider uploading reports for the commit f58dd3a to get more accurate results

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3967   +/-   ##
=====================================
  Coverage   82.0%   82.0%           
=====================================
  Files        171     171           
  Lines      12951   12955    +4     
=====================================
+ Hits       10627   10635    +8     
+ Misses      2104    2100    -4     
  Partials     220     220           

Impacted Files	Coverage Δ
sdk/metric/pipeline.go	89.6% <100.0%> (+0.1%)	⬆️

... and 2 files with indirect coverage changes