sdk/metric: Fix observable not registered error when the asynchronous instrument has a drop aggregation

[scorpionknifes]

Resolves #4666

When an observable instrument is dropped by view drop aggregation, it prints observable instrument not registered for callback message, as the instrument isn't register due to it being dropped.

opentelemetry-go/sdk/metric/meter.go

Lines 472 to 480 in cb8cb2d

	if _, registered := r.int64[oImpl.observablID]; !registered {
	global.Error(errUnregObserver, "failed to record",
	"name", oImpl.name,
	"description", oImpl.description,
	"unit", oImpl.unit,
	"number", fmt.Sprintf("%T", int64(0)),
	)
	return
	}

This PR attempts to ignore the above code by adding a flag to the observable instrument indicating it has been dropped and ignoring registered check

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (01472db) 82.3% compared to head (34653d9) 82.3%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4772   +/-   ##
=====================================
  Coverage   82.3%   82.3%           
=====================================
  Files        226     226           
  Lines      18406   18410    +4     
=====================================
+ Hits       15153   15157    +4     
  Misses      2969    2969           
  Partials     284     284           

Files	Coverage Δ
sdk/metric/instrument.go	100.0% <ø> (ø)
sdk/metric/meter.go	92.6% <100.0%> (+<0.1%)	⬆️

[pellared]

Just nit comments.

@MrAlias @MadVikingGod @dashpole, can you check if the way how it is fixed in meter.go and instrument.go is acceptable?