PMM-13575 use self label for agent metric

[idoqo]

PMM-13575

• Links to related pull requests (optional).

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Below we provide a basic checklist of things that would make it a good PR:

• Make sure to sign the CLA (Contributor License Agreement).
• Make sure all tests pass.
• Keep current with the target branch and fix conflicts if necessary.
• Update jira ticket description if necessary.
• Attach screenshots and/or console output to the jira ticket to confirm new behavior, if applicable.
• Leave notes to the reviewers if you need to focus their attention on something specific.

Once all checks pass and the code is ready for review, please add pmm-review-exporters team as the reviewer. That would assign people from the review team automatically. Report any issues on our Forum.