Fix no instances case for AgentCheck signature and add more tests

[AlexandreYang]

What does this PR do?

1. Add more tests for agent signature

2. Fix made to handle this case where instances is not passed when using agent 5 signature

AgentCheck('check_name', {'init_conf1': 'init_value1'}, {'agent_conf1': 'agent_value1'})
OpenMetricsBaseCheck('openmetrics_check', self.INIT_CONFIG, self.AGENT_CONFIG)

Motivation

Additional Notes

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #4784 into master will decrease coverage by 0.22%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
...g_checks_base/tests/test_openmetrics_base_check.py	100% <100%> (ø)	⬆️
...dog_checks_base/datadog_checks/base/checks/base.py	79.28% <100%> (ø)	⬆️
...tadog_checks/base/checks/openmetrics/base_check.py	87.23% <100%> (+2.78%)	⬆️
datadog_checks_base/tests/test_agent_check.py	99.39% <100%> (ø)	⬆️
vault/tests/common.py	85.71% <0%> (-14.29%)	⬇️
rabbitmq/tests/conftest.py	96.49% <0%> (-1.76%)	⬇️
vault/tests/test_vault.py	96.8% <0%> (-0.06%)	⬇️
windows_service/tests/common.py
oracle/tests/test_oracle.py
gearmand/datadog_checks/gearmand/gearmand.py
... and 172 more

[AlexandreYang]

In production, the AgentCheck is run by the datadog-agent, so, I guess the case should not be a real problem since the datadog-agent is probably always passing valid parameters.

Hence, setting this PR on HOLD to be merged later.

[hithwen]

Is this going to change to single instance in agent 8?

[AlexandreYang]

Yes, from my discussion with @ofek

But maybe should not mention the signature about Agent 8 in this PR, it's a bit out of scope. What do you think ? We can add it later.

[hithwen]

But it's mentioned here: https://github.com/DataDog/integrations-core/pull/4784/files#diff-874d5159f166811f6c78ff097251312cR123. Its ok to leave for later but I don't see a reason not to mention it if we know how its going to be

[AlexandreYang]

@ofek Is that certain that the signature for Agent 8 will like this ?

        Agent 8 signature:
            AgentCheck(name, init_config, instance)     # one instance
            AgentCheck.check()                          # no more instance argument for check method

[ofek]

That's the goal

Changes requested have been addressed