[exporter/kafkaexporter] Skip Username and Password config validation for IAM

[xavirg]

Description

In kafka.SASLConfig, Username and Password fields are always required:

func validateSASLConfig(c *kafka.SASLConfig) error {
	if c == nil {
		return nil
	}

	if c.Username == "" {
		return fmt.Errorf("auth.sasl.username is required")
	}

	if c.Password == "" {
		return fmt.Errorf("auth.sasl.password is required")
	}

However, when the authentication mechanism is set to either AWS_MSK_IAM or AWS_MSK_IAM_OAUTHBEARER, those properties are never used:

exporters:
  kafka/example:
    auth:
      sasl:
        username: "foo" # Not used
        password: "bar" # Not used
        mechanism: "AWS_MSK_IAM_OAUTHBEARER"
        aws_msk:
            region: us-east-1

Implemented logic to bypass User and Password validation when the mechanism is set to AWS IAM.

Testing

Added a test to make sure kafka.SASLConfig.Username and kafka.SASLConfig.Password are not required

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: xavirg / name: Javi Rodriguez (64ac675, eab1701, 9b11d06, 9ba8333, 5680e22)