Refactor unit tests to improve readability and maintainability
- Updated test assertions and mock setups in test_app.py for clarity.
- Added 'pod_id' to the metadata in test_ingestor.py for consistency.
- Introduced a new test for TAG_WRITTEN_COUNT in test_metrics.py to verify its definition.
- Enhanced test data structure in test_data_manager.py to include additional fields.
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