Examine source code of Better Page Ruler

{
  "manifest_version": 3,
  "name": "Better Page Ruler",
  "version": "0.0.2",
  "description": "Draw, resize, drag and move a ruler box on any webpage.",
  "permissions": [
    "scripting",
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "better-page-ruler@sayedulsayem.com",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "action": {
    "default_title": "Activate Ruler"
  },
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  }
}