Add schema_url to tracers

[tsloughter]

All instrumentation libraries in contrib should be required to get a tracer manually with the schema_url they are using for the attributes used in the telemetry.

[bryannaegele]

Provide access to or inject it someplace?

[tsloughter]

@bryannaegele provide a way to insert it. I think it may actually need to somehow go in either .app.src or the Application behaviour implementation as a schema_url() -> function since we create tracers for each application for the user.