Move metric.Descriptor out of metric API into sdkapi

[jmacd]

This is a simple move. I've added a single test of the new type.

Part of #2090.

[codecov]

Codecov Report

Merging #2269 (a3494aa) into main (66fccc0) will increase coverage by 0.0%.
The diff coverage is 87.0%.

@@          Coverage Diff          @@
##            main   #2269   +/-   ##
=====================================
  Coverage   72.5%   72.6%           
=====================================
  Files        168     169    +1     
  Lines      11874   11874           
=====================================
+ Hits        8611    8621   +10     
+ Misses      3029    3020    -9     
+ Partials     234     233    -1     

Impacted Files	Coverage Δ
attribute/type_string.go	40.0% <ø> (ø)
metric/metric_sdkapi.go	100.0% <ø> (ø)
metric/noop.go	30.7% <0.0%> (ø)
sdk/metric/processor/basic/basic.go	98.5% <ø> (ø)
sdk/metric/processor/reducer/reducer.go	100.0% <ø> (ø)
sdk/export/metric/metric.go	44.6% <40.0%> (ø)
sdk/metric/aggregator/aggregatortest/test.go	85.4% <57.1%> (ø)
metric/metrictest/meter.go	96.1% <60.0%> (ø)
bridge/opencensus/exporter.go	100.0% <100.0%> (ø)
exporters/otlp/otlpmetric/exporter.go	60.6% <100.0%> (ø)
... and 18 more

[jmacd]

I considered finish #2090 in one go, and it comes out a lot easier that way.