Support OTEL_METRIC_EXPORT_INTERVAL and OTEL_METRIC_EXPORT_TIMEOUT

[pellared]

Fixes #3731

[codecov]

Codecov Report

Merging #3763 (2edf7ad) into main (3d6a643) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3763     +/-   ##
=======================================
- Coverage   81.7%   81.6%   -0.1%     
=======================================
  Files        166     167      +1     
  Lines      12444   12459     +15     
=======================================
+ Hits       10168   10179     +11     
- Misses      2061    2065      +4     
  Partials     215     215             

Impacted Files	Coverage Δ
sdk/metric/reader.go	100.0% <ø> (ø)
sdk/metric/env.go	100.0% <100.0%> (ø)
sdk/metric/periodic_reader.go	91.6% <100.0%> (ø)
sdk/trace/batch_span_processor.go	80.2% <0.0%> (-1.8%)	⬇️

[dmathieu]

Trace environment variables are configured in sdk/internal/env, which is not trace specific.
Shouldn't we reuse this pattern for metrics, and therefore set that up in the same package?

[pellared]

Trace environment variables are configured in sdk/internal/env, which is not trace specific. Shouldn't we reuse this pattern for metrics, and therefore set that up in the same package?

I would rather tackle it "more globally". PTAL #3766