Investigate being able to reuse the OTLP input with the otel SDK structs

[dmathieu]

The OTLP input uses collector structs, as that's what we get from gRPC (example, metrics).
With elastic/apm-server#11470, we duplicate the same logic with the otel SDK structs.

Investigate refactoring the structs so we can remove that logic duplication (with no performance loss), maybe by converting the SDK structs into collector ones.