PoC for batching in PeriodicReader

[dashpole]

Batching logic is based on the collector's batchprocessor: https://github.com/open-telemetry/opentelemetry-collector/blob/587b90b9ecc1db959ee9104d5bf993591f80ca43/processor/batchprocessor/splitmetrics.go

PoC for open-telemetry/opentelemetry-specification#4895

This PR adds metric.WithMaxExportBatchSize to go.opentelemetry.io/sdk/metric, and causes the SDK to split batches before passing them to the exporter.

One potential issue is that I was hoping the export timeout could be applied individually to each batch, rather than to multiple serial export calls. But currently, we apply the timeout to collect + export. I've changed it to apply the timeout individually to collect, and to each export, but I'm curious how acceptable other maintainers think this kind of change would be. In practice, I suspect Collect() is not a notable source of latency or timeouts.

The spec for the timeout says:

exportTimeoutMillis - how long the export can run before it is cancelled. The default value is 30000 (milliseconds).

It would strike me as a bit odd if we did split metrics into batches to have the timeout still applied to the entire collect + multiple exports based on the spec.

[codecov]

Codecov Report

❌ Patch coverage is 57.59494% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.5%. Comparing base (64f28b0) to head (a93066b).
⚠️ Report is 64 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/metric/splitmetrics.go	48.8%	64 Missing and 1 partial ⚠️
sdk/metric/periodic_reader.go	93.5%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7930     +/-   ##
=======================================
- Coverage   81.7%   81.5%   -0.2%     
=======================================
  Files        304     305      +1     
  Lines      23283   23430    +147     
=======================================
+ Hits       19032   19116     +84     
- Misses      3864    3926     +62     
- Partials     387     388      +1     

Files with missing lines	Coverage Δ
sdk/metric/periodic_reader.go	86.9% <93.5%> (+1.5%)	⬆️
sdk/metric/splitmetrics.go	48.8% <48.8%> (ø)

... and 1 file with indirect coverage changes

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