Restore JSON support for metadata to metrics in 4.x

[tjquinno]

Environment Details

• Helidon Version: 4.x
• Helidon SE or Helidon MP
• JDK version:
• OS:
• Docker version (if applicable):

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Problem Description

The MP Metrics 5.0 spec removes the requirement for (and definition of) JSON format for metrics data and metadata.

Still, the JSON format prescribed by earlier MP metrics specs (and implemented in earlier releases of Helidon) is in some ways clearer and easier to work with than the Prometheus exposition format. Further, some of our tests use the JSON format.

PR #7139 for issue #5816 preserves JSON support for metrics data but not metadata. This issue represents restoring the JSON support for metadata.

[tjquinno]

Changes for this are in the big metrics PR.