declarative config: support for jmx metrics

[SylvainJuge]

The JMX metrics are captured with the JMX Insights component in instrumentation,
the metrics are defined in a YAML format.

Users can extend the provided metrics definitions by providing their own YAML files through the otel.jmx.config configuration option.

Those extra YAML configuration files are provided and managed by the end-user.

With the declarative configuration, we can configure the path to this custom metrics definitions.

However, it could also be interesting to nest the metrics definitions directly into the declarative configuration, hence allowing to configure everything with a single (but large) YAML file.

Doing so could help with the following use-cases:

• configure everything with a single file
• alternative to fetch the metrics definitions from a central remote location (config would be "pushed" instead of "pulled"), see #12775

There are however a few technical challenges:

• the current implementation relies on parsing directly the yaml file (but with the same parser library).
• we need to have an API/SPI that allows to delegate the parsing of declarative config to the JMX Insights component

[zeitlinger]

I can help with this task.

• should this be done after jmx idiomatic library API - part 1 #15220?
• anything else to know before trying it out?

[SylvainJuge]

• should this be done after jmx idiomatic library API - part 1 #15220?

I would say that yes, I also plan to work on #15196 which will also likely require changes to configuration, so ideally I would suggest to wait until #15196 is done to prevent lots of conflicts. This does not however prevent trying with current implementation in a draft PR to find the potential challenges.

• anything else to know before trying it out?

There are some aspects of the current parsing logic that we should probably improve, like how some attributes can (like units) can inherit a default value from the rule attributes.

One nice thing about this is that it could allow to implement declarative configuration in jmx-scraper and thus provide the ability to configure it with a single yaml file.