Do not block Temporality/Aggregation on OTLP metric export

[MrAlias]

Resolve #3925

Closes #4001

Currently for both the HTTP and gRPC OTLP metric exporters when Export is called all calls to Aggregation and Temporality are blocked. This means the whole SDK processing pipeline, which calls these methods, becomes blocked. This is especially bad when Export hangs indefinitely due to an inaccessible target.

Instead of having a universal client lock for a shared Exporter from the otlpmetric/internal package, have each exporter package implement a non-blocking Exporter directly. This removes the dependency on shared internal packages from an external module¹.

This does not completely remove the dependency on shared internal packages from an external module. That is left for a separate PR.

Footnotes

1. https://github.com/open-telemetry/opentelemetry-go/issues/3846 ↩

[codecov]

Codecov Report

Merging #4395 (37ba558) into main (528a0cb) will increase coverage by 0.1%.
The diff coverage is 88.8%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4395     +/-   ##
=======================================
+ Coverage   83.4%   83.5%   +0.1%     
=======================================
  Files        184     184             
  Lines      14380   14448     +68     
=======================================
+ Hits       11997   12071     +74     
+ Misses      2155    2147      -8     
- Partials     228     230      +2     

Files Changed	Coverage Δ
exporters/otlp/otlpmetric/internal/exporter.go	67.2% <ø> (ø)
...porters/otlp/otlpmetric/otlpmetricgrpc/exporter.go	83.0% <88.6%> (+35.4%)	⬆️
...porters/otlp/otlpmetric/otlpmetrichttp/exporter.go	83.0% <88.6%> (+35.4%)	⬆️
exporters/otlp/otlpmetric/otlpmetricgrpc/client.go	90.6% <100.0%> (+2.8%)	⬆️
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	78.6% <100.0%> (+1.0%)	⬆️

... and 1 file with indirect coverage changes

[MrAlias]

If merged, supersedes #4001

[pellared]

👍