Invenio-stats

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- introduce new field `updated_timestamp`` in the events and stats templates
and mappings
- improved calculation of aggregations skipping already aggregated events
- changed `refresh_interval` from 1m to 5s
- changed default events index name from daily to monthly
- moved BookmarkAPI to a new module

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- add extension method for building and caching queries

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- Upgrade to ``invenio-search`` 2.x
- Drop support for Elasticsearch 2, 5, and 6
- Add support for OpenSearch 1 and 2
- Drop support for Python 2.7 and 3.6
- Remove function ``invenio_stats.utils:get_doctype``
- Fix ``validate_arguments`` for query classes
- Add ``build_event_emitter`` function for creating an ``EventEmitter`` but not registering it as a signal handler
- Add ``ext.get_event_emitter(name) function for caching built ``EventEmitter`` objects per name
- Replace elasticsearch-specific terminology

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- add opensearch2 compatibility

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- Fix isort arguments
- Filter pytest deprecation warnings
- Set default values for metrics instead of None, when no index found

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- Removes Python 2.7 support.
- Centralizes Flask dependency via ``invenio-base``.