Support a global MeterProvider in go.opentelemetry.io/otel

[MrAlias]

Fixes #3816

The current support of a global MeterProvider is provided by go.opentelemetry.io/otel/metric/global. In the process of releasing the go.opentelemetry.io/otel/metric as stable, this functionality is being moved to go.opentelemetry.io/otel.

• Deprecate the existing go.opentelemetry.io/otel/metric/global. This will be removed in the v1.15.0 release after the release candidate phases publish this change.
• Move the error handling of the otel package to otel/internal/global. This is done to resolve the import cycle of otel/internal/metric depending on this error handling functionality.
• Copy the functionality from otel/metric/global into otel.
  • Note, the MeterProvider() function is renamed to GetMeterProvider() to match GetTracerProvider()
• Update the project to not use the deprecated otel/metric/global package anymore.

This approach wraps the existing functionality in otel/metric/global with the added functionality in otel. The opposite (moving the functionality to otel and wrapping otel/metric/global with otel) cannot be implemented without an import cycle (otel imports otel/metric and the wrap would mean otel/metric/global would import otel). Also, otel cannot depend on the underlying implementation in otel/metric/internal/global because that internal package is a sub-directory of otel.

[codecov]

Codecov Report

Merging #3818 (b51e31f) into main (c39e625) will decrease coverage by 0.1%.
The diff coverage is 63.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3818     +/-   ##
=======================================
- Coverage   81.7%   81.7%   -0.1%     
=======================================
  Files        167     170      +3     
  Lines      12453   12474     +21     
=======================================
+ Hits       10175   10192     +17     
- Misses      2063    2067      +4     
  Partials     215     215             

Impacted Files	Coverage Δ
metric/global/global.go	66.6% <ø> (ø)
metric/internal/global/instruments.go	60.8% <0.0%> (ø)
metric/internal/global/meter.go	93.8% <0.0%> (ø)
metric.go	66.6% <66.6%> (ø)
handler.go	100.0% <100.0%> (ø)
internal/global/handler.go	100.0% <100.0%> (ø)
sdk/trace/batch_span_processor.go	81.9% <0.0%> (+0.8%)	⬆️

[MrAlias]

Follow up to this PR: #3819