Properly validate only_custom_queries

[sarah-witt]

What does this PR do?

Properly validate metric groups and only_custom_queries

Motivation

previously when only_custom_queries was enabled, it errored:

__root__
  'dict' object is not callable
      Traceback (most recent call last):
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/base.py", line 1095, in run
          initialization()
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/base.py", line 489, in load_configuration_models
          instance_config = self.load_configuration_model(package_path, 'InstanceConfig', raw_instance_config)
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/base.py", line 533, in load_configuration_model
          raise_from(ConfigurationError('\n'.join(message_lines)), None)
        File "<string>", line 3, in raise_from
      datadog_checks.base.errors.ConfigurationError: Detected 1 error while loading configuration model `InstanceConfig`:
      __root__
        'dict' object is not callable

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• 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

[codecov]

Codecov Report

Merging #11992 (4ae1973) into master (9dfa1e7) will not change coverage.
The diff coverage is n/a.

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snowflake	94.85% <ø> (ø)

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