Clarify behaviour of enum processor without default or defined mapping (#6301)

plugins/processors/enum/README.md

@@ -25,8 +25,8 @@ source tag or field is overwritten.
     dest = "status_code"
 
     ## Default value to be used for all values not contained in the mapping
-    ## table.  When unset, the unmodified value for the field will be used if no
-    ## match is found.
+    ## table.  When unset and no match is found, the original field will remain 
+    ## unmodified and the destination tag or field will not be created.
     # default = 0
 
     ## Table of mappings
@@ -42,3 +42,9 @@ source tag or field is overwritten.
 - xyzzy status="green" 1502489900000000000
 + xyzzy status="green",status_code=1i 1502489900000000000
 ```
+
+With unknown value and no default set:
+```diff
+- xyzzy status="black" 1502489900000000000
++ xyzzy status="black" 1502489900000000000
+```

plugins/processors/enum/enum_test.go

@@ -123,3 +123,14 @@ func TestWritesToDestination(t *testing.T) {
 	assertFieldValue(t, "test", "string_value", fields)
 	assertFieldValue(t, 1, "string_code", fields)
 }
+
+func TestDoNotWriteToDestinationWithoutDefaultOrDefinedMapping(t *testing.T) {
+	field := "string_code"
+	mapper := EnumMapper{Mappings: []Mapping{{Field: "string_value", Dest: field, ValueMappings: map[string]interface{}{"other": int64(1)}}}}
+
+	fields := calculateProcessedValues(mapper, createTestMetric())
+
+	assertFieldValue(t, "test", "string_value", fields)
+	_, present := fields[field]
+	assert.False(t, present, "value of field '"+field+"' was present")
+}