OpenMetrics-format output missing EOF marker and using unexpected content type

[tjquinno]

Environment Details

• Helidon Version: 2.x, 3.x
• Helidon SE or Helidon MP both
• JDK version:
• OS:
• Docker version (if applicable):

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Problem Description

The MicroProfile Metrics spec describes JSON format and OpenMetrics format. https://download.eclipse.org/microprofile/microprofile-metrics-4.0/microprofile-metrics-spec-4.0.html#_openmetrics_format

It then links to a Prometheus format web page, which is very similar to OpenMetrics but not exactly the same.

Users have reported that the Helidon metrics output:

1. lacks the # EOF final line, and
2. emits text/plain rather than application/openmetrics-text.

Because of this, downstream Prometheus scrapers reject the output when --enable-feature=exemplar-storage is set.

Steps to reproduce

These steps are from a user description. Need to verify this.

1. Enable Helidon's exemplar support.
2. Set up Prometheus to scrape the Helidon service and store examplars.
   The Prometheus console for the service will display an brief, truncated error message about failing to scrape the service's metrics.

• Fix OpenMetrics formatting issue; add tests (2.x) #4901 for 2.x
• Fix OpenMetrics formatting error (3.x) #4900 for 3.x