otelslog is not reusing the pooled buffer

[pellared]

I noticed that this

opentelemetry-go-contrib/bridges/otelslog/handler.go

Line 357 in c80e464

buf.data = buf.data[:0:0]

"kills" the whole optimization gained from usage of sync.Pool because buf.data[:0:0] makes buf.data to never be really reused.

Demo: #5878

At the same time changing to buf.data[:0] does not work because of the recursive nature of the conversion.
We would need to call free() when everything is converted - e.g. between these lines:

opentelemetry-go-contrib/bridges/otelslog/handler.go

Lines 164 to 165 in c80e464

	h.logger.Emit(ctx, h.convertRecord(record))
	return nil

The other idea is to only use the pool for "top level attributes" (only in convertRecord).
However, the Logger.Emit documentation says:

The record may be held by the implementation. Callers should not mutate the record after passed

This would mean that we cannot pool the attributes as it can be held by the implementation.