Fix HTTP OTLPMetricExporter unit tests of `max_export_batch_size` to use real `export`

[tammy-baylis-swi]

Describe your environment

OS: macOS
Python version: Python 3.12.5
SDK version: 1.41.0.dev0
API version: 1.41.0.dev0

What happened?

The HTTP OTLPMetricExporter unit tests of max_export_batch_size don't use real export and mock a lot of things, reducing developer confidence. They're also not the most readable

Steps to Reproduce

Read the current OTLPMetricExporter test_split_metric_* functions.

Expected Result

See as few mocks used as possible for batch export testing

Actual Result

Oops, export is mocked

Additional context

Requested in #5015 that I add HTTP OTLPMetricExporter test coverage for max_export_batch_size with non-mocked export. Related also to #4577 and #5014

Similar to #5031

Would you like to implement a fix?

None

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