Move endpoint to copy common_metrics.yml to core

What does this MR do?

As discussed in #35228 (comment 257415731) cloning dashboards should be available for all tiers users. To achieve that this MR will move related code into core.

It also address #55261 (closed) missing file error, by switching to use absolute dashboard template path

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Separation of EE specific content

Reports #35228 (closed)