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

The OTLP input uses collector structs, as that's what we get from gRPC (example, [metrics](https://github.com/elastic/apm-data/blob/main/input/otlp/metrics.go)).
With https://github.com/elastic/apm-server/pull/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.