Prework

• Read and agree to the code of conduct and contributing guidelines.
• If there is already a relevant issue, whether open or closed, comment on the existing thread instead of posting a new issue.
• New features take time and effort to create, and they take even more effort to maintain. So if the purpose of the feature is to resolve a struggle you are encountering personally, please consider first posting a "trouble" or "other" issue so we can discuss your use case and search for existing solutions first.
• Format your code according to the tidyverse style guide.

Proposal

tar_progress_summary() should return a one-line data frame with the number of built, started, canceled, and errored targets. In addition, it should show columns to reflect when there was last a change to progress: one POSIXct value for the last modified time of _targets/meta/progress, and another column to show how long ago that was.