Remove dependency on metrics packages

[Aneurysm9]

Creates a new internal/metric module and moves metric-related internals there. The metric-related test helpers from oteltest have moved to metric/oteltest. This should eliminate all dependence on the metrics API module from modules that will be declared stable at v1.0.0.

[codecov]

Codecov Report

Merging #1988 (a25d3bd) into main (135ac4b) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #1988   +/-   ##
=====================================
  Coverage   77.2%   77.2%           
=====================================
  Files        161     162    +1     
  Lines       8495    8495           
=====================================
  Hits        6563    6563           
- Misses      1679    1680    +1     
+ Partials     253     252    -1     

Impacted Files	Coverage Δ
internal/global/state.go	100.0% <ø> (ø)
internal/metric/global/meter.go	90.3% <ø> (ø)
metric/global/metric.go	66.6% <ø> (-33.4%)	⬇️
metric/metrictest/meter.go	98.6% <ø> (ø)
internal/metric/global/metric.go	100.0% <100.0%> (ø)
internal/internaltest/alignment.go	0.0% <0.0%> (-37.5%)	⬇️
metric/config.go	91.6% <0.0%> (-8.4%)	⬇️
metric/metric_instrument.go	87.4% <0.0%> (-0.7%)	⬇️
metric/metric.go	51.9% <0.0%> (-0.6%)	⬇️
... and 2 more

[MrAlias]

Moving otel/oteltest to otel/metric/oteltest means that package name doesn't match the test package naming pattern we try to implement (i.e. ${PACKAGE_NAME}test). Can we rename it to metrictest?