[jmx-scraper] align custom configuration with instrumentation

[SylvainJuge]

This is a follow-up task of #1651

In instrumentation (JMX insights), the otel.jmx.config option provides the path to a list of custom YAML metrics definitions, we currently use otel.jmx.custom.scraping.config which does not allow to use a list.

Proposal

• rename otel.jmx.custom.scraping.config configuration option to otel.jmx.config
• add support for a list of configuration files
• providing compatibility with otel.jmx.custom.scraping.config is not required as this option only exists for jmx-scraper and is not expected to be widely used, the equivalent in jmx-gatherer was only supporting groovy.

[SylvainJuge]

I'm also volunteering to fix this so you can assign it to me, but I plan to work on it only when #1651 has been merged though.