[otlp-transformer] document experimental APIs

[pichlermarc]

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Description

@opentelemetry/otlp-transformer depends on @opentelemetry/sdk-logs and @opentelemetry/api-logs which may not be stable by the time we want to stabilize the @opentelemetry/otlp-transformer package.

Goal of this issue is to:

• mark all types and functions that use experimental types as @experimental

Prerequisites

This issue may be marked as up for grabs when all the following issues are done:

• #4583