feat: improve clarity of the source send cancelled error message

The `source send cancelled` does not immediately convey possible causes and does not provide actionable feedback.
Since this error most probably indicates backpressure-related errors, let’s be explicit about it.

Author: clementd-dd

changelog.d/source_send_cancelled_message_clarification.enhancement.md

@@ -0,0 +1,5 @@
+Updated the `source send cancelled` error message to point towards possible causes.
+This error usually happens either because a pipeline is shutting down or because
+of backpressure.
+
+authors: clementd-dd

lib/vector-core/src/source_sender/output.rs

@@ -76,7 +76,7 @@ impl Drop for UnsentEventCount {
             let _enter = self.span.enter();
             internal_event::emit(ComponentEventsDropped::<UNINTENTIONAL> {
                 count: self.count,
-                reason: "Source send cancelled.",
+                reason: "Source send interrupted mid-flight; pipeline may be overloaded or shutting down.",
             });
         }
     }