fix(instrument): Buffer counter underflowed (#23872)

Author: sialais

changelog.d/23872_buffer_counter_underflowed.fix.md

@@ -0,0 +1,3 @@
+Fix buffer counter underflowed, caused by the counter has not been updated(increase) timely when new event is coming.
+
+authors: sialais

lib/vector-buffers/src/topology/channel/sender.rs

@@ -204,6 +204,15 @@ impl<T: Bufferable> BufferSender<T> {
 
         let mut sent_to_base = true;
         let mut was_dropped = false;
+
+        if let Some(instrumentation) = self.instrumentation.as_ref()
+            && let Some((item_count, item_size)) = item_sizing
+        {
+            instrumentation.increment_received_event_count_and_byte_size(
+                item_count as u64,
+                item_size as u64,
+            );
+        }
         match self.when_full {
             WhenFull::Block => self.base.send(item).await?,
             WhenFull::DropNewest => {
@@ -214,6 +223,7 @@ impl<T: Bufferable> BufferSender<T> {
             WhenFull::Overflow => {
                 if let Some(item) = self.base.try_send(item).await? {
                     sent_to_base = false;
+                    was_dropped = true;
                     self.overflow
                         .as_mut()
                         .unwrap_or_else(|| unreachable!("overflow must exist"))
@@ -223,23 +233,9 @@ impl<T: Bufferable> BufferSender<T> {
             }
         }
 
-        if (sent_to_base || was_dropped)
-            && let (Some(send_duration), Some(send_reference)) =
-                (self.send_duration.as_ref(), send_reference)
-        {
-            send_duration.emit(send_reference.elapsed());
-        }
-
         if let Some(instrumentation) = self.instrumentation.as_ref()
             && let Some((item_count, item_size)) = item_sizing
         {
-            if sent_to_base {
-                instrumentation.increment_received_event_count_and_byte_size(
-                    item_count as u64,
-                    item_size as u64,
-                );
-            }
-
             if was_dropped {
                 instrumentation.increment_dropped_event_count_and_byte_size(
                     item_count as u64,
@@ -248,6 +244,12 @@ impl<T: Bufferable> BufferSender<T> {
                 );
             }
         }
+        if (sent_to_base || was_dropped)
+            && let (Some(send_duration), Some(send_reference)) =
+                (self.send_duration.as_ref(), send_reference)
+        {
+            send_duration.emit(send_reference.elapsed());
+        }
 
         Ok(())
     }