Merge branch 'unarchived-traces-metric' into 'master'

Track number of unarchived traces

Closes #49

See merge request gitlab-org/gitlab-monitor!89
2 jobs for master in 1 minute and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #4252754
rspec

00:01:02

passed #4252755
rubocop

00:01:00