Allow multi-instrument callbacks to be unregistered

[MrAlias]

Resolves #3452

• Add a new Registration type to the metric API and have the Meter's RegisterCallback method return one.
• Update implementations to support Registrations.

[codecov]

Codecov Report

Merging #3522 (5069a4f) into main (ca4cdfe) will increase coverage by 0.0%.
The diff coverage is 92.5%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3522   +/-   ##
=====================================
  Coverage   78.0%   78.0%           
=====================================
  Files        164     164           
  Lines      11759   11799   +40     
=====================================
+ Hits        9177    9214   +37     
- Misses      2387    2389    +2     
- Partials     195     196    +1     

Impacted Files	Coverage Δ
metric/noop.go	98.0% <66.6%> (-2.0%)	⬇️
metric/internal/global/meter.go	94.8% <89.1%> (-1.6%)	⬇️
sdk/metric/meter.go	87.2% <100.0%> (+0.2%)	⬆️
sdk/metric/pipeline.go	93.9% <100.0%> (+0.2%)	⬆️
exporters/jaeger/jaeger.go	91.1% <0.0%> (+0.8%)	⬆️

[MadVikingGod]

Is there a test showing that the callback isn't called in collects after Unregister()?

[MrAlias]

Is there a test showing that the callback isn't called in collects after Unregister()?

There is one for the global implementation. I'll add one for the SDK implementation.

[MrAlias]

Is there a test showing that the callback isn't called in collects after Unregister()?

There is one for the global implementation. I'll add one for the SDK implementation.

Added in c56c140

[jmacd]

Cool!