Restructure RegisterCallback method

[MrAlias]

Part of #3519

The RegisterCallback method is updated for ergonomic reasons to the following.

RegisterCallback(f Callback, instruments ...instrument.Asynchronous) (Registration, error)

The instruments are now passed as variadic arguments. This avoids the boilerplate of having to wrap instruments at call-time with a []instrument.Asynchronous{...} slice.

[codecov]

Codecov Report

Merging #3587 (2bd6671) into main (4286352) will not change coverage.
The diff coverage is 100.0%.

❗ Current head 2bd6671 differs from pull request most recent head 66bef3c. Consider uploading reports for the commit 66bef3c to get more accurate results

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3587   +/-   ##
=====================================
  Coverage   78.9%   78.9%           
=====================================
  Files        170     170           
  Lines      12460   12460           
=====================================
  Hits        9836    9836           
  Misses      2415    2415           
  Partials     209     209           

Impacted Files	Coverage Δ
metric/internal/global/meter.go	95.2% <100.0%> (ø)
metric/noop.go	62.1% <100.0%> (ø)
sdk/metric/meter.go	79.3% <100.0%> (ø)

[dashpole]

+1 This looks really nice.