processor_labels: Clarify an effect of absence of the target key when…

… insert and delete operations

Signed-off-by: Hiroshi Hatake <[email protected]>

pipeline/processors/labels.md

@@ -12,10 +12,10 @@ Classic mode configuration format doesn't support processors.
 
 | Key         | Description |
 | :---------- | :--- |
-| update  | Update an existing key with a value into metrics. The key-value pair is required. |
+| update  | Update an existing key with a value into metrics. The key-value pair is required. If the specified key does not exist, the operation is silently failed and no effect. |
 | insert  | Insert a new key with a value into metrics. The key-value pair is required. |
 | upsert  | Upsert a specific key with a value, the `upsert` operation will try to update the value of the key. If the key does not exist, the key will be created. The key-value pair is required. |
-| delete  | Delete a key from the labels of metrics. The key-value pair is required. |
+| delete  | Delete a key from the labels of metrics. The key-value pair is required. If the specified key does not exist, the operation is silently failed and no effect. |
 | hash    | Replace the key value with a hash generated by the SHA-256 algorithm from the specified label name, the binary value generated is finally set as an hex string representation. |