Examine source code of Metis

{
  "manifest_version": 3,
  "name": "Metis",
  "version": "0.1.2",
  "description": "A lightweight design tool for checking layouts in your browser. Measure elements, drop guides and see gaps.",
  "permissions": [
    "activeTab",
    "scripting"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "metis@frodowho",
      "strict_min_version": "140.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "commands": {
    "_execute_action": {
      "suggested_key": {
        "default": "Alt+Shift+M",
        "mac": "Alt+Shift+M"
      },
      "description": "Toggle the Measure toolbar"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "action": {
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  }
}