Add missing sentinel metrics to the docs

Add sentinel metrics to the Metrics - Terraform Cloud Agents docs NOTE: This needs the tfc-agent changes to be released before merging!!!!

Sentinel metrics are missing

If items do not apply to your changes, add (N/A) and mark them as complete.

[x] One or more labels describe the type of change (e.g. clarification) and associated product (e.g. tfc).
[x] Description links to related pull requests or issues, if any.

[x] Redirects have been added for moved, renamed, or deleted pages. This requires a separate PR in the terraform-website repository redirects.next.js file.
[x] API documentation and the API Changelog have been updated.
[x] Links to related content where appropriate (e.g., API endpoints, permissions, etc.).
[x] Pages with related content are updated and link to this content when appropriate.
[x] Sidebar navigation files have been updated for added, deleted, reordered, or renamed pages.
[x] New pages have metadata (page name and description) at the top.
[x] New images are 2048 px wide. They have HashiCorp standard annotation color (#F92672) and format (rectangle with rounded corners), blurred sensitive details (e.g. credentials, usernames, user icons), and descriptive alt text in the markdown for accessibility.
[x] New code blocks have the correct syntax and line breaks to eliminate horizontal scroll bars.
[x] UI elements (button names, page names, etc.) are bolded.
[x] The Vercel website preview successfully deployed.

[ ] I or someone else reviewed the content for technical accuracy.
[ ] I or someone else reviewed the content for typos, punctuation, spelling, and grammar.

hashicorp / terraform-docs-agents