Add information about setting lable value to n/a

Author: gnufied

contributors/design-proposals/volume-metrics.md

@@ -36,7 +36,9 @@ emitting these metrics.
 
 We will be using `HistogramVec` type so as we can attach dimensions at runtime. All
 the volume operation metrics will be named `storage_operation_duration_seconds`.
-Name of operation and volume plugin's name will be emitted as dimensions.
+Name of operation and volume plugin's name will be emitted as dimensions. If for some reason
+volume plugin's name is not available when operation is performed - label's value can be set
+to `<n/a>`.
 
 
 We are also interested in count of volume operation failures and hence a metric of type `NewCounterVec`