Refactor metric embeds to ease additional embed types

What does this MR do?

With #119016 (closed) and #40997 (closed), there will be two new EE GFM metrics filters. To ease the addition of these filters, this MR has the following changes:

• Moves inline_metrics_redactor_filter_spec.rb redactor tests into shared examples (to allow the new EE test to easily take advantage of them)
• Makes the xpath search in inline_metrics_filter.rb more specific, to prevent the logic in this class from running needlessly (as the current xpath search in this file will match the url formats as both embed types)
• Refactors permissions management in inline_metrics_redactor_filter.rb to allow for easier overrides and additions in a new EE file
• Refactors gfm_pipeline.rb to allow for easier overrides in the corresponding EE file
• Moves examples for both metrics filters into shared examples

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done