What does this PR do?

• Sets up a proper Metrics Guide landing page
  • Moves the existing "Custom Metrics Guides" page up one level in the left navigation. It now falls under Metrics, rather than Custom Metrics.
  • Renames "Custom Metrics Guides" to "Metrics Guides"
  • Creates sections in the "Metrics Guides" page for General Guides and Custom Metrics Guides
  • Adds the existing Metrics Guide "Calculating the system.mem.used metric" to the Metrics Guide page
  • Adds the existing "Custom Metrics Governance - Drop Metrics Missing Specific Tags" Guide to the Custom Metrics Guide section of "Metrics Guides".
• Migrates Dashboards FAQ "I'm switching between the sum/min/max/avg aggregators but the values look the same?"
  • Renames the file to be shorter. The new name is "different-aggregators-look-same.md"
  • Removes it from Dashboards FAQ page
  • Adds it to the Metrics Guide page
  • Updates links
• Deletes the out of date Metrics Guide "The Tag Configuration Cardinality Estimation Tool"
  • Deletes the file
  • Replaces links to "The Tag Configuration Cardinality Estimation Tool" to point to the API docs for the updated Tag Configuration Cardinality Estimator endpoint

Motivation

During the FAQ migration project, the Dashboards PM requested that several of the Dashboards FAQs move to the Metrics section. While moving the Dashboards FAQs to become Metrics Guides, I noticed some structural problems with the Metrics Guides section.

This PR fixes the structural problems in the Metrics Guides that I noticed and moves one of the Dashboards FAQs to be a Metrics Guide.

Structural problems:

• There was no top-level Metrics Guide Page. There was, however, a Custom Metrics Guide page.
• The Metrics Guide: "The Tag Configuration Cardinality Estimation Tool" describes an out of date endpoint. The tag configuration cardinality estimation endpoint is now public, and has a different interface than the one described in the guide.

Additional Notes

Reviewer checklist

• Review the changed files.
• Review the URLs listed in the Preview section.
• Check images for PII
• Review any mentions of "Contact Datadog support" for internal support documentation.