Update metrics dashboard base path

What does this MR do?

See #214282 (comment 376842850)

metrics-dashboard-base-path needs to be dynamic depending on the request path while we transition to the new metric dashboard route. This fixes that.

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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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