Metrics: Move the non-API types into sdkapi

[jmacd]

This is entirely changes of name with a few new tests, no functional changes.

The following interface types simply move:

Descriptor
MeterImpl
InstrumentImpl
SyncImpl
BoundSyncImpl
AsyncImpl
AsyncRunner
AsyncSingleRunner
AsyncBatchRunner

The following structs are moved:

Measurement
Observation

As the type structs are exposed to users, they have been given type aliases in the API.

The No-op implementations of sync and async instruments are no longer exported. New functions sdkapi.NewNoopAsyncInstrument() and sdkapi.NewNoopSyncInstrument() are provided instead.

This is a follow-on to #2197, which did all of the hard work in preparation for this change.

Fixes #2090.

[codecov]

Codecov Report

Merging #2271 (4cbf562) into main (3d4ae8d) will increase coverage by 0.0%.
The diff coverage is 76.3%.

@@          Coverage Diff          @@
##            main   #2271   +/-   ##
=====================================
  Coverage   72.5%   72.6%           
=====================================
  Files        168     170    +2     
  Lines      11874   11873    -1     
=====================================
+ Hits        8611    8621   +10     
+ Misses      3029    3019   -10     
+ Partials     234     233    -1     

Impacted Files	Coverage Δ
.../otlp/otlpmetric/internal/connection/connection.go	3.4% <0.0%> (-0.1%)	⬇️
...ers/otlp/otlpmetric/internal/otlpconfig/options.go	65.8% <ø> (ø)
...xporters/otlp/otlpmetric/otlpmetricgrpc/options.go	75.0% <0.0%> (-8.4%)	⬇️
...s/otlp/otlptrace/internal/connection/connection.go	16.2% <0.0%> (-0.3%)	⬇️
...ters/otlp/otlptrace/internal/otlpconfig/options.go	72.9% <ø> (ø)
exporters/otlp/otlptrace/otlptracegrpc/options.go	75.0% <0.0%> (-8.4%)	⬇️
metric/noop.go	100.0% <ø> (+69.2%)	⬆️
sdk/metric/processor/basic/basic.go	98.5% <ø> (ø)
sdk/metric/processor/reducer/reducer.go	100.0% <ø> (ø)
trace/config.go	63.7% <ø> (ø)
... and 26 more

[jmacd]

😀 🚀