Update Metric instrument names in InstrumentKind

[jmacd]

For the InstrumentKind enum in the metric package, this modernizes the instrument names.

Counter -> SyncCounter
UpDownCounter -> SyncUpDownCounter
SumObserver -> AsyncCounter
UpDownSumObserver -> AsyncUpDownCounter
ValueRecorder -> SyncHistogram
ValueObserver -> AsyncGauge

These names have been finalized in the Metrics SIG. This does not change the API in any other way, just updates these constant names.

[codecov]

Codecov Report

Merging #2089 (c8e61ad) into main (12f737c) will decrease coverage by 0.0%.
The diff coverage is 62.5%.

@@           Coverage Diff           @@
##            main   #2089     +/-   ##
=======================================
- Coverage   72.8%   72.8%   -0.1%     
=======================================
  Files        166     166             
  Lines       8168    8168             
=======================================
- Hits        5953    5949      -4     
- Misses      1987    1989      +2     
- Partials     228     230      +2     

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/instrumentkind_string.go	33.3% <ø> (ø)
metric/metric.go	51.9% <77.7%> (ø)
bridge/opencensus/exporter.go	100.0% <100.0%> (ø)
metric/metric_instrument.go	87.4% <100.0%> (ø)
sdk/export/metric/metric.go	36.8% <100.0%> (ø)
sdk/metric/aggregator/aggregator.go	100.0% <100.0%> (ø)
sdk/metric/selector/simple/simple.go	100.0% <100.0%> (ø)
sdk/metric/refcount_mapped.go	77.7% <0.0%> (-22.3%)	⬇️
... and 1 more

[jmacd]

I will re-open this following #2091.