Move global metric back to otel/metric/global for minor release

[MrAlias]

With the major changes we plan to release (#3973, #3971), it was decided at last week's SIG meeting that we will release them as another minor release and then go back into a stability RC cycle. This reverts the move of having the unstable metric API added to the stable global API.

[codecov]

Codecov Report

Merging #3986 (dd31d31) into main (5134557) will decrease coverage by 0.1%.
The diff coverage is 61.5%.

❗ Current head dd31d31 differs from pull request most recent head afeec40. Consider uploading reports for the commit afeec40 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3986     +/-   ##
=======================================
- Coverage   82.1%   82.1%   -0.1%     
=======================================
  Files        174     175      +1     
  Lines      12975   12975             
=======================================
- Hits       10655   10653      -2     
- Misses      2100    2102      +2     
  Partials     220     220             

Impacted Files	Coverage Δ
internal/global/state.go	100.0% <ø> (ø)
metric/internal/global/instruments.go	60.8% <0.0%> (ø)
metric/internal/global/meter.go	93.8% <0.0%> (ø)
metric/global/metric.go	66.6% <33.3%> (ø)
metric/internal/global/state.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes