Fix copy-paste error for noop SDK AsyncInstrument

[evantorrie]

As noted in #2370, the Noop SDK implementation of AsyncInstrument was returning the noopSyncInstrument.

This fixes the code to return the noopAsyncInstrument and also modifies the return type to AsyncImpl.

I marked this as Skip Changelog, but can add a Changelog entry if you consider it important enough.

[codecov]

Codecov Report

Merging #2376 (32efcaa) into main (249accd) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #2376   +/-   ##
=====================================
  Coverage   73.8%   73.8%           
=====================================
  Files        175     175           
  Lines      12436   12436           
=====================================
+ Hits        9182    9189    +7     
+ Misses      3017    3012    -5     
+ Partials     237     235    -2     

Impacted Files	Coverage Δ
metric/sdkapi/noop.go	61.5% <100.0%> (ø)
sdk/metric/sdk.go	81.6% <0.0%> (+1.4%)	⬆️
sdk/metric/refcount_mapped.go	100.0% <0.0%> (+20.0%)	⬆️