Add celery_worker_last_heartbeat_timestamp gauge

[roganartu]

Implements #228.

I tested this manually and it seems to work as expected:

❯ for i in {1..10}; do curl "http://localhost:9808/metrics" -sL | rg last_heartbeat_timestamp | rg -v '^#'; done
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.6771803171352026e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.6771803191380432e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.677180321139897e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.677180321139897e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.677180323141491e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.6771803251429496e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.6771803271442914e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.6771803291466177e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.6771803311494062e+09
celery_worker_last_heartbeat_timestamp{hostname="2f17101080ee"} 1.6771803331514475e+09

I cannot get the test I added to pass locally with the redis or rabbitmq brokers, however. I think it's something to do with my test setup, but it always fails at the is not None assertion, as (no matter how long the sleep is) there's never any samples. It passes fine with the memory broker.

[roganartu]

Abandoning as #235 fixed in a different way.