Add validation of metric values

Make sure to validate that when adding metric samples that the value is
a number.

Fixes: https://gitlab.com/gitlab-org/gitlab-exporter/issues/60
2 jobs for bjk/null_values in 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #5639268
rspec

00:00:45

passed #5639269
rubocop

00:00:51