Changes initial state for disabled prometheus integrations

What does this MR do?

Changes the initial empty state for the custom metrics when the prometheus integration has not been enabled

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49267, https://gitlab.com/gitlab-org/gitlab-ee/issues/8590

Does this MR meet the acceptance criteria?

Screenshots

Screen_Shot_2018-11-16_at_3.18.45_PM

Edited by Jose Ivan Vargas

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