Merge branch 'bjk/null_values' into 'master'

Add validation of metric values

Closes #60

See merge request gitlab-org/gitlab-exporter!102
2 jobs for master in 1 minute and 4 seconds (queued for 14 seconds)
Status Job ID Name Coverage
  Test
passed #5772442
rspec

00:00:44

passed #5772443
rubocop

00:00:49