Docs: improve readability of Prometheus metrics

What does this MR do?

Workaround to properly displayed the code on this table: https://docs.gitlab.com/ce/user/project/integrations/prometheus_library/kubernetes.html#metrics-supported

BEFORE	AFTER

Review apps:

• CE: http://ce-docs-fixes-prometheus-metrics-codeblock.178.62.207.141.xip.io/ce/user/project/integrations/prometheus_library/kubernetes.html#requirements
• EE: http://ce-docs-fixes-prometheus-metrics-codeblock.178.62.207.141.xip.io/ee/user/project/integrations/prometheus_library/kubernetes.html#requirements

Related issues

• Followup from https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/229
• For: https://gitlab.com/gitlab-com/gitlab-docs/issues/176

EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5133