如何使用python客户端库获取Pubsub中存在的未送达消息(指标api)的数量？

from google.cloud import pubsub_v1

publisher = pubsub_v1.PublisherClient()

project_id = "your-project-id"
topic_id = "your-topic-id"
subscription_id = "your-subscription-id"

topic_path = publisher.topic_path(project_id, topic_id)
subscription_path = publisher.subscription_path(project_id, subscription_id)

response = publisher.get_topic_subscriptions(request={"topic": topic_path})
subscription_names = response.subscriptions

request = pubsub_v1.types.ListSubscriptionsRequest(
    project=project_id,
    topic=topic_path,
    subscription=subscriptions,
    filter_='delivery_attempted = false'
)

response = publisher.list_subscriptions(request=request)
num_unacked_messages = 0

for subscription in response:
    num_unacked_messages += subscription.num_unacked_messages

print("Number of undelivered messages:", num_unacked_messages)