[OTel] Dynamic Sampling Context propagation does not work for nested spans

[tonyo]

Extracted from #537.

Problem

Dynamic Sampling Context propagation from nested spans doesn't work properly in OpenTelemetry integration.
Main reason: no publicly exposed mechanism to get the transaction (as in, a root span) for the given nested span.
At the moment we're doing this, meaning that if we're doing a cross-service request from a nested span, the DSC won't be properly set.

Potential Solutions

• Expose a method on Span that allows to get the root span (transaction) of the given span.