Make OtelSpan public instead of package-private

[ThomasVitale]

The OtelSpan class implementing the Span interface is package private, which makes it not possible to act on the underlying OpenTelemetry Span object (unless Reflection is used). One use case for this would be to add structured events to a Span (necessary until micrometer-metrics/micrometer#5238 is implemented).

That is different from the BraveSpan class, which is set as public.

Is it possible to make OtelSpan public in the 1.3.x release line?

[ThomasVitale]

In case the feature request is accepted, I drafted a PR to align the visibility of OtelSpan to the one of BridgeSpan. #809