How to get Metadata for SourceGenerator - Documenation

[FlorianGrimm]

Hi
Is there more documentation (except the source code) how analyzers query their data.
To explain what I mean (- not to get a solution)
I want to I want to write a SourceGenerator. My missing piece is how to get the metadata. Only using Attributes will not work. I want to use a class/method as definition - like AspNetCore Startup combined with an (fluent) configuration like AutoMappers projection definition with lambda expressions. I try to use DataFlowAnalysis or ExpressionEvaluator, but I found not so much documentation.

So my problem to use roslyn SourceGenerator (and roslyn-analyzers) is the documentation how to use it correctly.