Properly validate credentials API key before using

[sarah-witt]

What does this PR do?

• Address Datadog-Plugin breaks on LTS v2.319.1 w/ built-in node update #266
• Only use credentials API Key if not blank, and use the same findSecret method when validating API key and selecting the API key to use

Description of the Change

Alternate Designs

Possible Drawbacks

Verification Process

• Install old version of plugin
• Enter API key as a Secret
• Update plugin to new version
• Ensure metrics are still submitting without user intervention

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• 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 one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.