style guide corrections to migrate metrics doc

[JTorreG]

Proposed changes

Update new document to follow the style guide:

• Not using sentence case for title
• Inconsistently using Platform Metrics or Platform metrics capitalization, same for Custom Metrics
• Missing blank lines after titles
• Missing F5 on first mention of the product
• "Click" instead of "Select"
• Extra blank spaces at the end of the lines
• Other markdown issues (blank lines, missing)

Checklist

Before merging a pull request, run through this checklist and mark each as complete.

• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have rebased my branch onto main
• I have ensured my PR is targeting the main branch and pulling from my branch from my own fork
• I have ensured that the commit messages adhere to Conventional Commits
• I have ensured that documentation content adheres to the style guide
• If the change involves potentially sensitive changes¹, I have assessed the possible impact
• If applicable, I have added tests that prove my fix is effective or that my feature works
• I have ensured that existing tests pass after adding my changes
• If applicable, I have updated README.md and CHANGELOG.md

Footnotes

1. Potentially sensitive changes include anything involving code, personally identify information (PII), live URLs or significant amounts of new or revised documentation. Please refer to our style guide for guidance about placeholder content. ↩

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