Function flush_with_retry 

async fn flush_with_retry(
    name: &str,
    cfg: &BatchConfig,
    flusher: &Arc<dyn BatchFlusher>,
    buffer: &mut Vec<Value>,
)

Flushes buffer, retrying per cfg on failure. Honours first_fail by dropping the delivered head and retrying only the tail. Entries arriving during a retry sleep simply queue in the channel and form the next batch. When retries are exhausted, the remaining entries are dropped with a tracing::error!.