It already exists but it is very expensive...   it is just like they don't want you to be able to measure 5G radiation on a commercial level before it is too late...

Here is a list of what is available in mid-2019:

https://en.wikipedia.org/wiki/Personal_RF_safety_monitor

Wavecontrol SMP2 + WPF40 field probe - 40Ghz $3000~

https://img.directindustry.com/images_di/photo-g/58470-9065864.jpg

Wawemon RF40 - 40Ghz  $1400~

Nardalert S3 - 50Ghz $1700~

EME Guard - 40Ghz $1800~