Add added_by and last_used fields to keys

[chapurlatn]

Fixes #2717 by adding fields and accessors

[google-cla]

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[gmlewis]

According to: https://www.githubstatus.com/incidents/52z0j6phhnjs
there was a GitHub Actions outage that prevented tests from being run on this PR.
Manually restarting them now.

[gmlewis]

Unable to manually restart the jobs. Hoping GitHub tech support will unblock the pending GitHub Actions that currently appear to be zombies.

[gmlewis]

Closing and reopening... hoping to re-trigger the stuck GitHub Actions...

[gmlewis]

Reopening...

[gmlewis]

Bravo - tests are now running.

[gmlewis]

Thank you, @chapurlatn !
LGTM.
Merging.

[codecov]

Codecov Report

Merging #2718 (804f078) into master (3135011) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2718   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files         131      131           
  Lines       11459    11459           
=======================================
  Hits        11236    11236           
  Misses        152      152           
  Partials       71       71           

Impacted Files	Coverage Δ
github/users_keys.go	100.00% <ø> (ø)

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