Deprecate (in 4.x) and remove (in 5.x) integration for the Prometheus metrics API

[tjquinno]

Task Description

Helidon provides a neutral metrics API and an implementation based on Micrometer which provides Prometheus-format (and JSON) output.

Prior to introducing the neutral API and the Micrometer-based implementation, Helidon created an integration library for the Prometheus metrics API. See:

• https://prometheus.github.io/client_java/
• https://github.com/prometheus/client_java/tree/simpleclient

We receive very few support queries about this integration which implies it has low usage.

This epic lists the tasks for deprecating and removing this integration.

• Mark the Prometheus integration as deprecated (doc and code) in 4.x
• Remove the integration in 5.0