Fix race in helpers/prometheus/log_hook.go: Fire() method

What does this MR do?

Fixes races that can occur with concurrent calls to Fire() in helpers/prometheus/log_hook.go.

Why was this MR needed?

https://gitlab.com/gitlab-com/infrastructure/issues/1079#note_22260206

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

assigned to @tmaczukin

added 1 commit

• 800aed05 - Replace mutex with sync.atomic usage

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The test seems to be quite reliable. 100 repeats without the fix fails each time, 100 repeats with the fix succeeds each time.

assigned to @ayufan

added 1 commit

• cfb6dbc2 - Replace mutex with sync.atomic usage

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added 1 commit

• b06bfc5b - Replace mutex with sync.atomic usage

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assigned to @tmaczukin

resolved all discussions

added 1 commit

• e27af8a7 - Replace mutex with sync.atomic usage

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@ayufan Updated!

assigned to @ayufan

enabled an automatic merge when the pipeline for e27af8a7 succeeds

merged

mentioned in commit b4c159dc

mentioned in commit 8e829463