Harden Prometheus client wrapper

What does this MR do?

Harden Prometheus client wrapper further against broken "block" given.

In the current usage we don't have any risks, this will make future implementations safe

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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