Update Prometheus to 2.8.1
Update Prometheus to 2.8.1 to follow omnibus-gitlab's Prom version as Prometheus 2.8.1 is out.
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Upstream update (omnibus-gitlab): omnibus-gitlab!3182 (merged)
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Previous update in this repo: !363 (merged)
Closes #432 (closed)
Replaces !567 (closed)