Define whether instrumentations can emit non-event log records

[pellared]

Imagine I'm instrumenting kafka:

• there could be some events I'd emit - e.g. cluster leader is unreachable - which are documented in the semconv
• there could also be things I just want to let my users know of via logs. E.g. certain configuration was picked, connection has dropped, etc. I could use 3rd party logging library, but why if I can use OTel logging API without adding a new dependency?

The library logs are emitted with is not relevant to users - they'd get the same logs if lib used 3rd party logger or OTel API. So I don't see why we should limit instrumentations from using OTel API to emit undocumented regular (but structured) logs.

_Originally posted by @lmolkova in https://github.com/open-telemetry/opentelemetry-specification/pull/4225#discussion_r1781723278_

[lmolkova]

To add a bit of context: client libraries or infra components usually emit some logs using some non-OTel logging API. If they are instrumented natively with OTel they need to have 2 dependencies - OTel and 3rd party logging lib.

While it's unlikely that existing libraries will replace their existing logging with OTel one, by providing user-facing logging API we allow future native instrumentations that use OTel for metrics and tracing to also use OTel logging API instead of adding a new dependency.

Before #4225 it was not possible.