[ENG-6264] add monthly "item usage" report

[aaxelb]

Purpose

compute monthly "usage" metrics for public items on osf

Changes

• add osf.metrics.reports.PublicItemUsageReport (elasticsearch metric document model)
• add osf.metrics.reporters.public_item_usage.PublicItemUsageReporter (monthly reporter)
• in osf.metrics.counted_usage, add get_item_type and get_provider_id for consistent reuse with the new reporter

QA Notes

Please make verification statements inspired by your code and what your code touches.

• Verify
• Verify

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