The general answer is yes, the google-services.json is safe to check in to your repo and is something that should be shared among engineers on your team. The JSON file does not contain any super-sensitive information (like a server API key). It does contain some information like your database URL, Android API key, and storage bucket. These are not secrets, but if your security rules are not set up correctly an attacker could use them against you. However since these values are also compiled into your APK as resources, the theoretical attacker would not need your JSON file to get these values anyway.
For a library or open-source sample we do not include the JSON file because the intention is that users insert their own to point the code to their own backend. That's why you won't see JSON files in most of our firebase repos on github.