[SDK] Update MetricProducer interface to match spec

[punya]

Following https://github.com/open-telemetry/opentelemetry-specification/blob/v1.36.0/specification/metrics/sdk.md#metricproducer.

Updates #1831

Changes

Updates MetricProducer to return a batch of metric points instead of receiving a callback.

• According to the spec, Produce SHOULD also take a Resource parameter, but I have chosen not to do that (because existing implementations pull the resource from the metric context). I'm not familiar with this area so I'd appreciate any advice.

• MetricFilter support isn't stable yet so I've kept it out of this initial PR.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.78%. Comparing base (497eaf4) to head (9d4f849).
Report is 133 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/src/metrics/state/metric_collector.cc	66.67%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3044      +/-   ##
==========================================
+ Coverage   87.12%   87.78%   +0.66%     
==========================================
  Files         200      195       -5     
  Lines        6109     5954     -155     
==========================================
- Hits         5322     5226      -96     
+ Misses        787      728      -59     

Files with missing lines	Coverage Δ
...opentelemetry/sdk/metrics/export/metric_producer.h	100.00% <ø> (ø)
...opentelemetry/sdk/metrics/state/metric_collector.h	100.00% <ø> (ø)
sdk/src/metrics/metric_reader.cc	78.79% <100.00%> (ø)
sdk/src/metrics/state/metric_collector.cc	92.60% <66.67%> (-0.26%)	⬇️

... and 121 files with indirect coverage changes

[marcalff]

Thanks for the fix.

Not a full review, just some preliminary comments.

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[marcalff]

LGTM, thanks for the fix.