Add chunk flush outcome "Dropped", for short series (#2998)

If the series is below a configured length it gets dropped instead of
flushed.  Add a separate `outcome` for this case, otherwise it is
reported as "NoChunksToFlush".

Signed-off-by: Bryan Boreham <[email protected]>

Co-authored-by: Marco Pracucci <[email protected]>

Author: bboreham

pkg/ingester/flush.go

@@ -111,6 +111,7 @@ const (
 	noSeries
 	noChunks
 	flushError
+	reasonDropped
 	maxFlushReason // Used for testing String() method. Should be last.
 )
 
@@ -138,6 +139,8 @@ func (f flushReason) String() string {
 		return "NoChunksToFlush"
 	case flushError:
 		return "FlushError"
+	case reasonDropped:
+		return "Dropped"
 	default:
 		panic("unrecognised flushReason")
 	}
@@ -307,12 +310,17 @@ func (i *Ingester) flushUserSeries(flushQueueIndex int, userID string, fp model.
 					"queue", flushQueueIndex,
 				)
 				chunks = nil
+				reason = reasonDropped
 			}
 		}
 	}
 
 	userState.fpLocker.Unlock(fp)
 
+	if reason == reasonDropped {
+		return reason, nil
+	}
+
 	// No need to flush these chunks again.
 	for len(chunks) > 0 && chunks[0].flushed {
 		chunks = chunks[1:]