feat(vhost-user-blk): added config_change_time_us metric

Added `config_change_time_us` metric to the vhost-user-block device.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

src/vmm/src/devices/virtio/vhost_user_block/device.rs

@@ -256,6 +256,8 @@ impl<T: VhostUserHandleBackend> VhostUserBlockImpl<T> {
     }
 
     pub fn config_update(&mut self) -> Result<(), VhostUserBlockError> {
+        let start_time = utils::time::get_time_us(utils::time::ClockType::Monotonic);
+
         // This buffer is used for config size check in vhost crate.
         let buffer = [0u8; BLOCK_CONFIG_SPACE_SIZE as usize];
         let (_, new_config_space) = self
@@ -273,6 +275,9 @@ impl<T: VhostUserHandleBackend> VhostUserBlockImpl<T> {
             .trigger_irq(IrqType::Config)
             .map_err(VhostUserBlockError::IrqTrigger)?;
 
+        let delta_us = utils::time::get_time_us(utils::time::ClockType::Monotonic) - start_time;
+        self.metrics.config_change_time_us.store(delta_us);
+
         Ok(())
     }
 }