fix: replaced getJSON with transform.Unmarshal

[lamATnginx]

Proposed changes

• Replaced getJSON with transform.Unmarshal in dimensions.html and metrics.html

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[nginx-jack]

Looks to be rendering the same as before 👍