Redo OTLPMetricExporter unit tests of max_export_batch_size to use real export

[tammy-baylis-swi]

Description

Full redo of the HTTP OTLPMetricExporter unit tests of max_export_batch_size because they weren't using real export. It should be real OTLPMetricExporter.export now while Session.post is mocked at the HTTP level. The old tests also weren't the most readable, so I've factored out a helper to generate the metrics_data exported.

Fixes #5035

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Removed old tests
• Replaced with new tests
• tox -e py312-test-opentelemetry-exporter-otlp-proto-http

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated