[celery] patch TaskRegistry to support old-style task with ddtrace-run

[palazzem]

Overview

After solving #465 #469 to fix #423, we discovered that ddtrace-run wasn't instrumenting old-style tasks. The issue was related to the fact that celery.task.Task (Task base class) wasn't instrumented by our system; unfortunately, it wasn't possible to instrument it directly because of how the base class is used by Celery. Indeed, a direct instrumentation of celery.task.Task ended with the following exception:

[2018-06-11 12:37:50,595: ERROR/MainProcess] Received unregistered task of type u'_get_exec_options'.
The message has been ignored and discarded.

Did you remember to import the module containing this task?
Or maybe you're using relative imports?

Please see
http://docs.celeryq.org/en/latest/internals/protocol.html
for more information.

The full contents of the message body was:
'[[], {"stop": true}, {"chord": null, "callbacks": null, "errbacks": null, "chain": null}]' (89b)
Traceback (most recent call last):
  File "[...]/celery/worker/consumer/consumer.py", line 561, in on_task_received
    strategy = strategies[type_]
KeyError: u'_get_exec_options'

With this patch, we properly patch a Task when it's registered instead of doing it at import time.

[alxch-]

Very good idea to directly patch register 👍