Function write 

pub(super) async fn write(
    handler: PromStoreProtocolHandlerRef,
    batcher: Option<Arc<LogicalTablePendingRowsBatcher>>,
    metric_engine: bool,
    samples: Vec<(QueryContextRef, RowInsertRequests)>,
    histograms: Vec<(QueryContextRef, RowInsertRequests)>,
    exemplars: ExemplarRequests,
    ctx: QueryContextRef,
) -> Outcome

Preflights all destinations together, retaining per-kind write progress.