WIP Metric API refactor

[MrAlias]

Related to and possibly fixes:

• Asynchronous Instrument API non-compliant with OTel Specification: creation does not accept callbacks #3451
• Asynchronous Instrument API non-compliant with OTel Specification: callbacks cannot be unregistered #3452
• Asynchronous Instrument API does not follow OTel Specification recommendation on naming #3453
• There MUST NOT be any API for creating an instrument other than with a Meter #3454

[codecov]

Codecov Report

Merging #3456 (29d1452) into main (484c8bd) will decrease coverage by 0.5%.
The diff coverage is 69.1%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3456     +/-   ##
=======================================
- Coverage   77.6%   77.1%   -0.6%     
=======================================
  Files        164     164             
  Lines      11651   11614     -37     
=======================================
- Hits        9050    8957     -93     
- Misses      2392    2450     +58     
+ Partials     209     207      -2     

Impacted Files	Coverage Δ
metric/config.go	7.9% <0.0%> (-14.8%)	⬇️
metric/instrument/config.go	0.0% <ø> (ø)
sdk/metric/instrument.go	83.3% <ø> (ø)
sdk/metric/pipeline.go	88.6% <38.4%> (-4.9%)	⬇️
sdk/metric/instrument_provider.go	66.6% <70.3%> (-13.4%)	⬇️
metric/internal/global/instruments.go	54.3% <72.2%> (ø)
metric/internal/global/meter.go	86.1% <81.2%> (-10.4%)	⬇️
metric/noop.go	92.3% <96.9%> (-7.7%)	⬇️
sdk/metric/meter.go	100.0% <100.0%> (ø)
... and 5 more

[MadVikingGod]

How do you practically use this WithCallback?

ctr := meter.Int64ObservableCounter("stuff", WithCallback(func(ctx context.Context){
ctr.Observe(ctx, 1) // Where is ctr defined? ctr will only be available after this clojure is defined.
}))

[MrAlias]

opentelemetry-go/metric/example_test.go

Lines 54 to 66 in 29d1452

	var memoryUsage metric.Int64ObservableGauge
	memoryUsage, err := meter.Int64ObservableGauge(
	"MemoryUsage",
	metric.WithUnit(unit.Bytes),
	metric.WithCallback(func(ctx context.Context) error {
	// Do Work to get the real memoryUsage.
	// mem := GatherMemory(ctx)
	mem := 75000
	memoryUsage.Observe(ctx, int64(mem))
	return nil
	}),
	)