In order to count the visitors of their websites, Google Analytics is not necessary, often these analysis tools are installed by their agency, but never find consideration.

To see if an ad showed the necessary benefit, a simple log analysis tool fed from the weblogs of the web server is enough, usually no unsightly cookie banners are needed here. 

Ask your web host if an analysis tool is already available for your website, for example the following tools:

Opensource: AWStats - Open Source Log File Analyzer for advanced statistics (GNU GPL)

Matomo Analytics - The Google Analytics alternative that protects your data