在R中组合两行缺少值

I have a dataset with id and measurements taken at time. Some measurements are taken at time0 while some others are taken at time1. That results in some missing values. I want to combine rows with time0 and time 1 since both are baseline measurements and a new dataset has time starting from 1.Basically merge time0 and time1 for each id. Cannot think of a way to do that. To show what my data looks like, here is some simulated data. I have a dataset with id and measurements taken