Change virtual threads metrics default to disabled; remove count-leve enable/disable; update tests and doc

[tjquinno]

Description

Resolves #9652

Release Note

Joe - this release note supersedes all earlier ones for virtual threads metrics.

---

Helidon's built-in meters now include new ones for additional thread information.

New meters related to virtual thread usage (disabled by default)

• current number of active virtual threads
• total number of virtual thread starts
• total number of pinned threads
• timer (with histogram) of pinned threads' durations
• number of failed attempts to submit virtual threads to platform threads

For performance reasons Helidon by default does not report the virtual threads meters.

New meter related to platform threads

Helidon also now exposes the new system meter thread.starts which displays the total number of platform thread starts performed in the JVM since server start-up.

Configuration

• metrics.virtual-threads.enabled

  Whether the virtual thread meters feature is enabled.

  Default: false

• metrics.virtual-threads.pinned.threshold

  Helidon tracks pinned thread events only if the duration reaches a threshold. This value is a Duration expression.

  Default: PT0.02S (20 ms)

---

Overview of the PR changes

• The virtual threads meters are now disabled by default.
• There is no separate setting for enabling the virtual thread count meters.
• Test changes.
• Doc changes.
• Revert earlier change to mp-1. It contains a test verifying how many base meters are registered. With virtual threads meters now disabled by default, the expected value goes from 1 back to 0. (I'd changed this in an earlier PR for virtual threads meters.)

Documentation

Includes updates to the SE and MP metrics guide pages and the generated doc for config.