Flatten sdk/metric/aggregation into sdk/metric

[MrAlias]

Resolve #4432

• Move the Aggregation interface and its implementations to sdk/metric
  • Unexport all methods of the Aggregation interface
  • Prepend Aggregation to all concrete implementations of the Aggregation interface
• Deprecate the sdk/metric/aggregation pkg
• Decouple sdk/metric/internal/aggregate from the aggregation types so there is no import cycle
• Stop using the previously exported methods of Aggregator in the metric exporters
  • There is no real need validate the aggregation selector these exporters receive
• Update all use of the deprecated sdk/metric/aggregation with sdk/metric

[codecov]

Codecov Report

Merging #4435 (424c234) into main (199dc34) will increase coverage by 0.0%.
Report is 1 commits behind head on main.
The diff coverage is 79.2%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4435   +/-   ##
=====================================
  Coverage   78.8%   78.9%           
=====================================
  Files        253     254    +1     
  Lines      20668   20650   -18     
=====================================
- Hits       16307   16305    -2     
+ Misses      4012    4000   -12     
+ Partials     349     345    -4     

Files Changed	Coverage Δ
exporters/otlp/otlpmetric/internal/exporter.go	67.2% <0.0%> (ø)
sdk/metric/aggregation/aggregation.go	69.0% <ø> (-12.0%)	⬇️
sdk/metric/instrument.go	92.0% <ø> (ø)
sdk/metric/pipeline.go	85.9% <51.6%> (-2.6%)	⬇️
sdk/metric/aggregation.go	88.8% <88.8%> (ø)
...xporters/otlp/otlpmetric/internal/oconf/options.go	83.5% <100.0%> (+3.6%)	⬆️
...porters/otlp/otlpmetric/otlpmetricgrpc/exporter.go	83.0% <100.0%> (ø)
...pmetric/otlpmetricgrpc/internal/oconf/envconfig.go	93.1% <100.0%> (ø)
...tlpmetric/otlpmetricgrpc/internal/oconf/options.go	87.8% <100.0%> (+3.7%)	⬆️
...porters/otlp/otlpmetric/otlpmetrichttp/exporter.go	83.0% <100.0%> (ø)
... and 11 more

... and 1 file with indirect coverage changes

[pellared]

I approved as I am going on PTO and the comments to be addressed are trivial.

[dashpole]

I like the move overall. Sum -> AggregationSum needs to be fixed

[MrAlias]

Package site index:

This naming strategy accomplishes the intended grouping of aggregations in docs.