class Google::Auth::ServiceAccountJwtHeaderCredentials

def apply!(a_hash, opts = {})

The jwt token is used as the value of a 'Bearer '.

hash.
Construct a jwt token if the JWT_AUD_URI key is present in the input 
def apply!(a_hash, opts = {})
  jwt_aud_uri = a_hash.delete(JWT_AUD_URI_KEY)
  return a_hash if jwt_aud_uri.nil?
  jwt_token = new_jwt_token(jwt_aud_uri, opts)
  a_hash[AUTH_METADATA_KEY] = "Bearer #{jwt_token}"
  a_hash
end