Simplify Prometheus metric collection

[09Catho]

Summary

• replace the Prometheus collector's metrics deque with a single current metrics snapshot
• translate the collected snapshot directly during Prometheus collection
• keep target_info generation and repeated scrape behavior unchanged

Fixes #2500.

Testing

• PYTHONPATH='I:\OSC\repos\opentelemetry-python\exporter\opentelemetry-exporter-prometheus\src;I:\OSC\repos\opentelemetry-python\opentelemetry-sdk\src;I:\OSC\repos\opentelemetry-python\opentelemetry-api\src;I:\OSC\repos\opentelemetry-python\tests\opentelemetry-test-utils\src' python -m pytest exporter\opentelemetry-exporter-prometheus\tests\test_prometheus_exporter.py -q
• python -m ruff check exporter\opentelemetry-exporter-prometheus\src\opentelemetry\exporter\prometheus_init_.py exporter\opentelemetry-exporter-prometheus\tests\test_prometheus_exporter.py

Note: I also ran test_entrypoints.py with local PYTHONPATH; the entrypoint import test failed because the package entry point is not installed in this ad hoc environment, while the other 3 tests passed.

[linux-foundation-easycla]

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[Copilot]

Pull request overview

This PR refactors the Prometheus exporter’s collection pipeline by replacing the internal queued MetricsData buffering with a single “current snapshot”, and translating that snapshot directly during Prometheus collect().

Changes:

• Replace _CustomCollector’s deque of pending MetricsData with a single _metrics_data snapshot.
• Simplify collect() to translate a single snapshot and yield once per scrape.
• Preserve cached target_info creation behavior (created once and reused).

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[herin049]

Closing this PR as already addressed by #5123