Examine source code of Mailtrack for Gmail PM

{
  "manifest_version": 2,
  "name": "Mailtrack for Gmail (Self-Hosted)",
  "version": "1.3",
  "description": "Adds an optional tracking pixel to Gmail messages and shows a simple open dashboard. Requires your own tracking URL/server.",
  "permissions": [
    "storage",
    "activeTab",
    "https://mail.google.com/*",
    "<all_urls>"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "js": [
        "content_gmail.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_title": "Mailtrack Dashboard",
    "default_icon": "icons/logo_128.png",
    "default_popup": "popup.html"
  },
  "options_ui": {
    "page": "options.html",
    "open_in_tab": false
  },
  "icons": {
    "32": "icons/logo_32.png",
    "64": "icons/logo_64.png",
    "128": "icons/logo_128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{00f532df-d228-4ad4-9e01-64b17e591252}"
    }
  }
}