Move InstrumentKind into the new metric/sdkapi package

[jmacd]

This moves the InstrumentKind type into the metric/sdkapi package, to remove it from user-facing documentation in the metric package.

Part of #2090.

[codecov]

Codecov Report

Merging #2091 (d5d7360) into main (1cb5cdc) will not change coverage.
The diff coverage is 73.9%.

@@          Coverage Diff          @@
##            main   #2091   +/-   ##
=====================================
  Coverage   72.6%   72.6%           
=====================================
  Files        176     177    +1     
  Lines      12000   12000           
=====================================
  Hits        8719    8719           
  Misses      3043    3043           
  Partials     238     238           

Impacted Files	Coverage Δ
...rs/otlp/otlpmetric/internal/otlpmetrictest/data.go	0.0% <0.0%> (ø)
...tlp/otlpmetric/internal/otlpmetrictest/otlptest.go	0.0% <0.0%> (ø)
metric/metric_instrument.go	86.4% <ø> (-1.3%)	⬇️
metric/sdkapi/instrumentkind_string.go	40.0% <ø> (ø)
metric/metric.go	56.4% <80.0%> (ø)
bridge/opencensus/exporter.go	100.0% <100.0%> (ø)
metric/sdkapi/instrumentkind.go	100.0% <100.0%> (ø)
sdk/export/metric/metric.go	41.6% <100.0%> (ø)
sdk/metric/aggregator/aggregator.go	100.0% <100.0%> (ø)
sdk/metric/aggregator/aggregatortest/test.go	85.4% <100.0%> (ø)
... and 2 more