Function sample_to_csv_row 

pub fn sample_to_csv_row(s: &Sample, interval_secs: u64) -> String

Serialize a Sample as a single CSV row (no newline).

interval_secs is required to convert bytes/sec rates into per-interval byte counts, matching Python resource-tracker’s convention.

Process fields not yet collected are emitted as empty strings. All process fields are empty when no PID is being tracked.