Don't set values when pattern doesn't match in regex processor (#4396)

(cherry picked from commit 35d2f90)

plugins/processors/regex/regex.go

@@ -67,15 +67,19 @@ func (r *Regex) Apply(in ...telegraf.Metric) []telegraf.Metric {
 	for _, metric := range in {
 		for _, converter := range r.Tags {
 			if value, ok := metric.GetTag(converter.Key); ok {
-				metric.AddTag(r.convert(converter, value))
+				if key, newValue := r.convert(converter, value); newValue != "" {
+					metric.AddTag(key, newValue)
+				}
 			}
 		}
 
 		for _, converter := range r.Fields {
 			if value, ok := metric.GetField(converter.Key); ok {
 				switch value := value.(type) {
 				case string:
-					metric.AddField(r.convert(converter, value))
+					if key, newValue := r.convert(converter, value); newValue != "" {
+						metric.AddField(key, newValue)
+					}
 				}
 			}
 		}

plugins/processors/regex/regex_test.go

@@ -222,16 +222,15 @@ func TestNoMatches(t *testing.T) {
 			},
 		},
 		{
-			message: "Should emit empty string when result_key given but regex doesn't match",
+			message: "Should not emit new tag/field when result_key given but regex doesn't match",
 			converter: converter{
 				Key:         "request",
 				Pattern:     "not_match",
 				Replacement: "x",
 				ResultKey:   "new_field",
 			},
 			expectedFields: map[string]interface{}{
-				"request":   "/users/42/",
-				"new_field": "",
+				"request": "/users/42/",
 			},
 		},
 	}