Add mechanism to execute setup steps before the first check run

[ofek]

Motivation

We need a way to run things after the check_id is set by the Agent, such as submitting configuration metadata

[codecov]

Codecov Report

Merging #4713 into master will increase coverage by 0.7%.
The diff coverage is 95.65%.

Impacted Files	Coverage Δ
...dog_checks_base/datadog_checks/base/checks/base.py	78.59% <100%> (+2.51%)	⬆️
datadog_checks_base/tests/test_agent_check.py	99.28% <94.59%> (-0.72%)	⬇️
postgres/tests/common.py
vertica/tests/metrics.py
vertica/datadog_checks/vertica/views.py
riakcs/datadog_checks/riakcs/riakcs.py
zk/tests/test_e2e.py
win32_event_log/tests/test_check.py
...service/datadog_checks/windows_service/__init__.py
mcache/datadog_checks/mcache/mcache.py
... and 191 more

[therve]

That works, but about having a check_initializations method that we call once, with a flag set after it's called?

[ofek]

@therve The thing is there could be multiple such functions

[therve]

@therve The thing is there could be multiple such functions

Sure, just call them from the check_initialization method?

[AlexandreYang]

Seems this would have a similar effect, if I'm not missing something.

            while self.check_initializations:
                self.check_initializations[0]()
                self.check_initializations.pop(0)

[ofek]

@therve That means users/we would have to override that method, call super, then finally add the real logic. The current way allows us to have appropriately named functions that do exactly one thing and just append it to check_initializations.