[OTLP] Modernize internal contracts - utilize Spans

[Kielek]

Hi @paulomorgado , buffer is used to store the serialized protobuf message. Currently all existing APIs such as WriteStringWithTag, WriteDoubleWithTag are also using byte[]. If we want to change this, I think we can create a dedicated PR later? @rajkumar-rangaraj What do you think?

Isn't this method being added now? Why add to the "problem" instead of adding to the "solution"?

Have a look at my branch.

Originally posted by @paulomorgado in #6534 (comment)

For more details, see discussion in the linked PR.