If a glass of cold water wakes baby up, I would suggest taking a bottle of cold water with you.
then the baby may be still enough for the tech to get some good measurements etc while baby isn't so active.
Then you could have your cold drink and wake baby up.
Maybe call ahead and ask if that would be OK.
I've seen some people post before that they couldn't get measurements or see xyz because baby was hyper active that day.
So maybe you can get the best of both worlds?