async fn prepare(
handler: &PromStoreProtocolHandlerRef,
samples: Vec<(QueryContextRef, RowInsertRequests)>,
histograms: Vec<(QueryContextRef, RowInsertRequests)>,
exemplars: ExemplarRequests,
ctx: QueryContextRef,
) -> Result<(Vec<(usize, QueryContextRef, RowInsertRequests)>, HashSet<(String, String, String)>)>