unicorn_workers is returning zero

On August 29, the unicorn_workers metric started reading zero.

This is having a knock on effect for several other metrics and alerts on GitLab.com.

https://thanos-query.ops.gitlab.net/graph?g0.range_input=1w&g0.expr=sum(unicorn_workers%7Benv%3D%22gprd%22%2C%20environment%3D%22gprd%22%7D)%20by%20(fqdn)&g0.tab=0

https://thanos-query.ops.gitlab.net/graph?g0.range_input=2w&g0.expr=gitlab_component_saturation%3Aratio%7Benv%3D%22gprd%22%2C%20component%3D%22workers%22%7D&g0.tab=0

The most recent change I can see to the unicorn sampler was at least one month prior, so not sure if its related https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30506

cc @rcobb @ayufan @jprovaznik