That's an application (as is Docker, also built in Go), but the question was about internal Google services and... we don't know because company secrets, but it's likely on the rise as it was written as a replacement for C++ which was their previous main language for backend services alongside Java/Kotlin. One source with the charming name "assbuttass" [0] says all new services are written in Go, with a follow-up by "deathmaster99" saying only 10% of code is Go, but this was a year ago and even 10% at Google's scale probably represents tens of millions of LOC.

[0] https://www.reddit.com/r/golang/comments/1c9fhet/how_much_go...