Function prepare_remote_write_context 

fn prepare_remote_write_context(
    params: &RemoteWriteQuery,
    query_ctx: QueryContext,
    remote_write_version: &str,
) -> (String, QueryContext, HistogramTimer)

Returns the context still un-shared so the caller can attach request-derived extensions (the v2 per-table metadata index) before wrapping it in an Arc.