Fix for delayed global registration

[MadVikingGod]

Fixes: #2777

What happened to cause this?

Before the global package, the only way to create an instrument was via the actual SDK. The global package introduced the idea of a delegated instrument, which when set forwarded all calls to the underlying instrument. But part of the SDK checks that the instruments passed to RegisterCallback were created by this SDK. The problem is that it does this poorly (eg just that they are of an internal type, so it won't guard against 2 instances of SDK interchanging instruments.)

So when you SetGlobalMeterProvider is called, it delegates all of the instruments and callbacks to the provided MP, but it will call RegisterCallback with a delegated type not the SDK type (it's stored inside the delegate).

How does this fix the problem?
Added an internal Unwrap to the delegated types, and before calling RegisterCallback it will unwrap the delegates so the SDK's type should be called.

[codecov]

Codecov Report

Merging #2784 (44a365a) into main (57a248d) will decrease coverage by 0.1%.
The diff coverage is 39.5%.

@@           Coverage Diff           @@
##            main   #2784     +/-   ##
=======================================
- Coverage   75.9%   75.7%   -0.2%     
=======================================
  Files        178     178             
  Lines      11665   11707     +42     
=======================================
+ Hits        8855    8871     +16     
- Misses      2585    2611     +26     
  Partials     225     225             

Impacted Files	Coverage Δ
metric/internal/global/instruments.go	57.2% <13.3%> (-9.3%)	⬇️
metric/internal/global/meter.go	96.4% <100.0%> (+0.2%)	⬆️

[lucasoares]

Tested and this PR solves #2777.