Add ability for users to configure settings related to virtual threads meters

[tjquinno]

Description

Resolves #9652

Release Note

Joe - this release note supersedes and expands on the one for the earlier PR #9619 for virtual threads metrics.

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

New meters related to virtual thread usage

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

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 features is enabled.

  Default: true

• metrics.virtual-threads.count.enabled

  The virtual thread count meters are disabled by default for performance reasons. Enable them by setting metrics.virtual-threads.count.enabled=true in configuration, but be aware doing so can degrade the server's performance.

  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)

Documentation

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