ServiceAccountCredentials tokens are not renewed automatically

[leandrogoe]

Environment details

• OS: macOS 10.15.7
• Ruby version: 2.6.1
• Gem name and version: 0.14.0

Steps to reproduce

1. Create a service account and a cloud run service
2. Grant roles/run.invoker to your service account
3. Generate a key-file for your service account
4. Create your credentials object, using your key-file and you service url:
   credentials = Google::Auth::ServiceAccountCredentials.make_creds(json_key_io: File.open(keyfile), target_audience: service_url)
5. Invoke your service using the headers generated by credentials.apply!({})
6. This works! Now wait for an hour+.
7. Generate new headers by calling credentials.apply!({}) again and call your service.
8. It fails with an authorization error.

Background

Normally I would not expect the gem to handle the automatic renewal of the token, but since I found that there's already code to renew it automatically I presume this is a bug from the gem.

Thanks in advance!