chore(sinks): refactor RequestBuilder/RequestMetadata to streamline splitting/building

[tobz]

As mentioned in #12755 (comment here), there's a good chance to streamline the building of request metadata when in RequestBuilder, as there's typically very little unique data that needs to be extracted from a batch of events. Instead, most metadata boils down to the number of events in a batch, their aggregate size, encoded size, finalizers, and so on.

Right now, RequestMetadata exclusively handles event count/byte size-related metrics for the purpose of driving EventsSent/BytesSent downstream when a request is being sent. However, more opportunities exist, such as also encapsulating finalizers, as well as the actually unique metadata for the given RequestBuilder, such that we can minimize the necessary boilerplate for a RequestBuilder implementation down to the bare essentials.