ProtoBuffer, BoundedBuffer refactoring placeholder #4482

Pre-filing checklist

• I searched existing issues and didn't find a duplicate

Component(s)

Rust OTAP dataflow (rust/otap-dataflow/)

Description

Placeholder for code cleanup in crates/pdata/src/otlp/common.rs; around 50% of this code is related to protocol buffer encoding with limited buffers, and it's a mess.

Additional Context

See code review feedback in #4450.