Function write_prometheus_rows_with_progress 

async fn write_prometheus_rows_with_progress(
    prom_store_handler: PromStoreProtocolHandlerRef,
    pending_rows_batcher: Option<Arc<PendingRowsBatcher>>,
    prom_store_with_metric_engine: bool,
    batches: Vec<(QueryContextRef, RowInsertRequests)>,
) -> Result<PromWriteOutcome, PromWriteError>

Writes preflighted PRW batches and keeps the number of persisted rows on error.

The v2 handler uses that partial progress to return Prometheus’ written sample/histogram headers even when a later table write fails.