Add invalid builds counter metric to stage seeds class

Grzegorz Bizonrequested to merge
metrics/gb/add-tmp-stage-creation-metrics into master

What does this MR do?

This merge requests add a temporary metric to collect more info on invalid builds that are being created without stage_id assigned, that we see on production.

What are the relevant issue numbers?

See gitlab-org/gitlab-ce#38756

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