class Google::Auth::GCECredentials

def fetch_access_token _options = {}
  query, entry =
    if token_type == :id_token
      [{ "audience" => target_audience, "format" => "full" }, "service-accounts/default/identity"]
    else
      [{}, "service-accounts/default/token"]
    end
  query[:scopes] = Array(scope).join "," if scope
  begin
    log_fetch_query
    resp = Google::Cloud.env.lookup_metadata_response "instance", entry, query: query
    log_fetch_resp resp
    case resp.status
    when 200
      build_token_hash resp.body, resp.headers["content-type"], resp.retrieval_monotonic_time
    when 403, 500
      raise Signet::UnexpectedStatusError, "Unexpected error code #{resp.status} #{UNEXPECTED_ERROR_SUFFIX}"
    when 404
      raise Signet::AuthorizationError, NO_METADATA_SERVER_ERROR
    else
      raise Signet::AuthorizationError, "Unexpected error code #{resp.status} #{UNEXPECTED_ERROR_SUFFIX}"
    end
  rescue Google::Cloud::Env::MetadataServerNotResponding => e
    log_fetch_err e
    raise Signet::AuthorizationError, e.message
  end
end