Function wait_for_all_datanode_online

pub async fn wait_for_all_datanode_online(
    greptime: MySqlPool,
    timeout: Duration,
)

Waits until all datanodes are online within a specified timeout period.

This function repeatedly checks the status of all datanodes and waits until all of them are online or the timeout period elapses. A datanode is considered online if its active_time is less than 3 seconds.