Add experimental support for batching in periodic reader

[dashpole]

Adds experimental support for maxExportBatchSize using the OTEL_GO_X_METRIC_EXPORT_BATCH_SIZE=<size> environment variable.

Previous prototype: #7930

This preserves existing behavior for timeouts when batching is not used, but individually applies the timeout to export calls when batching is used.

[codecov]

Codecov Report

❌ Patch coverage is 98.06452% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.2%. Comparing base (cdf74d8) to head (1011b26).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/metric/periodic_reader.go	88.0%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8071     +/-   ##
=======================================
+ Coverage   82.0%   82.2%   +0.1%     
=======================================
  Files        308     309      +1     
  Lines      24060   24211    +151     
=======================================
+ Hits       19742   19904    +162     
+ Misses      3940    3928     -12     
- Partials     378     379      +1     

Files with missing lines	Coverage Δ
sdk/metric/internal/x/x.go	100.0% <100.0%> (+100.0%)	⬆️
sdk/metric/splitmetrics.go	100.0% <100.0%> (ø)
sdk/metric/periodic_reader.go	85.9% <88.0%> (-1.2%)	⬇️

... and 1 file with indirect coverage changes

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[MrAlias]

Looks good overall. Only issue is is the timeout leaking into direct Collect(ctx, rm) calls.

[MrAlias]

Looks good overall. Only minor suggestion on changelog entry.