This is effectively a request to include download-related information available to PyPI when interacting with the index server, so that informtion can used to make ecosystem-wide decision (by querying said information via https://warehouse.pypa.io/api-reference/bigquery-datasets.html#download-statistics-table).

https://github.com/pypi/linehaul-cloud-function is the PyPI side implementation. https://github.com/pypa/pip/blob/24.0/src/pip/_internal/network/session.py#L109 is the pip side implementation.

This data powers decision making such as https://pypistats.org/packages/__all__ (and similar sites), https://mayeut.github.io/manylinux-timeline/ and a few ad-hoc queries to determine usage patterns across the ecosystem.