fix: Fix kafka exporter instance argument

[nilforoosh]

Brief description of Pull Request

When there is only one URI in kafka-exporter, the instance parameter is ignored and there is a bug.

Pull Request Details

Issue(s) fixed by this Pull Request

Notes to the Reviewer

PR Checklist

• Documentation added
• Tests updated
• Config converters updated

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[dehaansa]

To make this a bit more readable, should we refactor to have a guard clause checking if Instance is set, and otherwise check the KafkaURIs?

if c.Instance != "" {
  return c.Instance, nil
}

//check KafkaURIs here

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