[SRU] Watcher crashes on creation of multiple audits and gets stuck in PENDING

[ Impact ]

  * The watcher releases targeted by this SRU are experiencing a bug where you can only create one audit of type CONTINUOUS. Any subsequently created audits end up getting stuck in a pending state. The root cause of this error is the conversion of an improperly typed date which causes watcher to crash. The function converting the date format, utc_timestamp_to_datetime, expects the timestamp to be of type float but Watcher has been passing the date as a decimal object. The patch at [1] correctly typecasts to float before converting to a datetime object

  * The commit landed upstream in 2024.2.

[ Test Plan ]

  * Deploy openstack yoga on jammy with watcher and gnocchi services

  * Create two watcher audits of CONTINUOUS type and monitor their status
    openstack optimize audit create --name test_audit_1 -s workload_stabilization -g workload_balancing --audit_type CONTINUOUS --interval 60 --auto-trigger
    openstack optimize audit create --name test_audit_2 -s workload_stabilization -g workload_balancing --audit_type CONTINUOUS --interval 60 --auto-trigger
    openstack optimize audit list

  * Without the patch, the second audit will get stuch in state PENDING and systemctl status watcher-decision-engine.service reveals that a crash occured. With the patch, both audits successfully enter a state of "ONGOING"

[ What can go wrong ]

  * This commit overrides the apscheduler's implementation of get_next_run_time, since the apscheduler's implementation obtains the decimal.Decimal object which crashes the engine. This should expand compatibility to include SQLAlchemy 2.0 but may have otherwise have effects. It shouldn't since the function it's overriding is what precipitates the issue but it may affect legacy software (eg. older SQLAlchemy)

[1] https://opendev.org/openstack/watcher/commit/d6f169197efc5b4f6c8a2e6bc38177b0641ca05c

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Original Description:

A customer is facing an issue where the watcher-decision-engine service crashes when creating an audit plan with the Audit type set to CONTINUOUS. Below are the steps to reproduce the issue:

Environment Details:
1. Deploy Openstack Yoga on Jammy with Watcher and Gnocchi as watcher's storage backend

2. Create an audit
openstack optimize audit create --name workload_stabilization_test_1 -s workload_stabilization -g workload_balancing --audit_type CONTINUOUS --interval 60 --auto-trigger

3. Check the audit state
openstack optimize audit list
Observe it says "CONTINUOUS ONGOING"

4. Create a second audit
openstack optimize audit create --name workload_stabilization_test_2 -s workload_stabilization -g workload_balancing --audit_type CONTINUOUS --interval 60 --auto-trigger

5. Check the audit state
openstack optimize audit list
Observe the second audit is stuck in "CONTINUOUS PENDING"

6. Check watcher's status and observe that it crashed with the following traceback
systemctl status watcher-decision-engine.service

Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: self.run()
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: File "/usr/lib/python3.10/threading.py", line 953, in run
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: self._target(*self._args, **self._kwargs)
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: File "/usr/lib/python3/dist-packages/apscheduler/schedulers/blocking.py", line 32, in _main_loop
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: wait_seconds = self._process_jobs()
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: File "/usr/lib/python3/dist-packages/apscheduler/schedulers/base.py", line 1006, in _process_jobs
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: jobstore_next_run_time = jobstore.get_next_run_time()
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: File "/usr/lib/python3/dist-packages/apscheduler/jobstores/sqlalchemy.py", line 84, in get_next_run_time
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: return utc_timestamp_to_datetime(float(next_run_time))
Nov 27 19:53:54 juju-2752e1-86-lxd-27 watcher-decision-engine[965896]: TypeError: float() argument must be a string or a real number, not 'NoneType'

This was fixed upstream in 2024.2 at https://opendev.org/openstack/watcher/commit/d6f169197efc5b4f6c8a2e6bc38177b0641ca05c which properly addresses the type conversion