Google fit had this convenient menu entry named "Log your weight", which is something I do about every day for a few years.
I'm using the Android app Libra as well. It is actually much better than Google Fit at weight tracking thanks to adequate data smoothing, giving a better idea on how your weight is evolving.
Google Fit uses your weight for calories calculations however so it's a good idea to keep it up to date.
But Google removed the "Log your weight" access in latest Fit app update.