Schema should allow to express which resource attributes need to be associated with a signal

[lmolkova]

Process and system metrics are expected to be attached to a corresponding resource - #1215

Currently there is no good way to express it in a formal way:

• documenting them as regular attributes could be confusing and lead to collisions when the same attribute is recorded as a regular metric attribute AND as a resource attribute
• not documenting them in yaml would be limiting for any automation (md or code generation, validation, etc)