Idea: support classpath-based analysis

# Problem

Current strategy for finding documentation providers relies on analyzing the actual src code under a folder. This work for the service-specific metrics.
However, often metrics are provided by external JARs - as in the case a spring boot-based applications.
As an application owner, it would be nice to be able to compile a documentation containing all the metrics that can be potentially generated by the application so it can be delivered to customers.

# Implementation ideas

Just throwing out some ideas. Im not much familiar yet with the metrics documentation strategy, so apologies if the ideas doesnt make sense.

## Build time analysis

Find all documentation provider implementations (e.g ObservationDocumentation), and produce the objects by reflection. Additionally, if the sources are provided in the mvn repo, the javadoc could also be looked up on the dep-sources.jar.

## Runtime analysis

Same as above, but done at runtime. Applications can also expose endpoints (such as spring boot actuators) for producing the docs.

The issue names no files, tests, or entry points. Start by reading the existing metrics documentation strategy and locating how documentation providers are discovered from source code. Compare the proposed build-time and runtime approaches, then define a supported analysis path and a testable form of generated documentation.

Written by the indexing model from the issue text.