Summary:
Previously, after the metadata refactor, only upid worked as a ctx key to access properties such as pod_name.
However, other keys such as pod_id in the network_stats table should also be able to produce values such as pod_name.
This diff cleans up the remnants of the _attr-based logic and treats upid and other keys such as pod id consistently.

Test Plan: ran a query to do df.ctx['pod'] on network_stats table which didn't work before, existing unit tests.

Reviewers: philkuz, jamesbartlett, #engineering

Reviewed By: philkuz, #engineering

JIRA Issues: PP-2100

Differential Revision: https://phab.corp.pixielabs.ai/D5869

GitOrigin-RevId: 2529c2ff32be4e80d3c022bf5cb1099ca8911356