Logging schema key

[adriangb]

A lot of users will be piping their logs to a backend with logs from a lot of other systems. They'll likely need to set up some sort of log processing pipeline that only applies to OTEL logs. But how do they know what logs are OTEL logs? I propose that we add an optional a schema identifier to logs that can be used to identify the log data model / schema and it's version. E.g. {"OTLSchema": "0.1.0"}.