Use display_default as a fallback for default when validating config models

[ian28223]

What does this PR do?

Use display_default instead of default when validating models.

Example

In kafka_consumer check, default for the optional kafka_client_api_version is None but an example value is shown as 2.3.0. However, the generated defaults.py currently takes the value from default > then example > then falls back to NO_DEFAULT aka <KAFKA_CLIENT_API_VERSION>

• spec.yaml - https://github.com/DataDog/integrations-core/blob/7.30.x/kafka_consumer/assets/configuration/spec.yaml

     - name: kafka_client_api_version
       ...
       value:
         type: string
         example: "2.3.0"
         display_default: null
  

• conf.yaml.example - https://github.com/DataDog/integrations-core/blob/7.30.x/kafka_consumer/datadog_checks/kafka_consumer/data/conf.yaml.example#L56-L65

      ## @param kafka_client_api_version - string - optional
      ## Specify the highest client protocol version supported by all brokers in the cluster.
      ...
      # kafka_client_api_version: 2.3.0
  

• defaults.py

  • Expected:

    def instance_kafka_client_api_version(field, value):
          return None
    

  • Actual: https://github.com/DataDog/integrations-core/blob/7.30.x/kafka_consumer/datadog_checks/kafka_consumer/config_models/defaults.py#L31-L32

    def instance_kafka_client_api_version(field, value):
          return '2.3.0'
    

Example 2: https://github.com/DataDog/integrations-core/pull/9699/files#r672222987

Motivation

#8593

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