4.x: DOC for Helidon neutral metrics API

[tjquinno]

Environment Details

• Helidon Version: 4.x
• Helidon SE or Helidon MP
• JDK version:
• OS:
• Docker version (if applicable):

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Problem Description

Related to #4868

Documentation for the new Helidon metrics API needs to be written and it needs to replace the previous SE metrics doc. PR #7261 contains some information but the doc impact is much too large to capture in the PR alone.

(There will be some impact on the MP metrics doc as well but that will be handled as part of a separate issue when we convert our MP metrics implementation to use the new Helidon API.)

Tim will work with Lisa to create this doc.