add async to gtag script

Author: drbruced12

conf.py

@@ -126,6 +126,11 @@ def get_version():
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
+html_css_files = [
+    'css/rtd_theme_mods.css',
+    ]
+
+
 # HTML Favicon
 html_favicon = '_static/bridge.ico'
 
@@ -259,8 +264,5 @@ def get_version():
 
 def setup(app):
 
-    app.add_css_file('css/rtd_theme_mods.css')
-
-
-    app.add_js_file('https://www.googletagmanager.com/gtag/js?id=G-K101Q1MWLM')
+    app.add_js_file('https://www.googletagmanager.com/gtag/js?id=G-K101Q1MWLM', loading_method="async")
     app.add_js_file(None, body=GA_INVOKE_JS)