Allow regexp processor to mix different tags (#5863)

plugins/processors/regex/README.md

@@ -2,6 +2,8 @@
 
 The `regex` plugin transforms tag and field values with regex pattern. If `result_key` parameter is present, it can produce new tags and fields from existing ones.
 
+For tags transforms, if `append` is set to `true`, it will append the transformation to the existing tag value, instead of overwriting it.
+
 ### Configuration:
 
 ```toml

plugins/processors/regex/regex.go

@@ -18,6 +18,7 @@ type converter struct {
 	Pattern     string
 	Replacement string
 	ResultKey   string
+	Append      bool
 }
 
 const sampleConfig = `
@@ -70,6 +71,11 @@ func (r *Regex) Apply(in ...telegraf.Metric) []telegraf.Metric {
 		for _, converter := range r.Tags {
 			if value, ok := metric.GetTag(converter.Key); ok {
 				if key, newValue := r.convert(converter, value); newValue != "" {
+					if converter.Append {
+						if v, ok := metric.GetTag(key); ok {
+							newValue = v + newValue
+						}
+					}
 					metric.AddTag(key, newValue)
 				}
 			}

plugins/processors/regex/regex_test.go

@@ -108,6 +108,20 @@ func TestTagConversions(t *testing.T) {
 				"resp_code": "2xx",
 			},
 		},
+		{
+			message: "Should append to existing tag",
+			converter: converter{
+				Key:         "verb",
+				Pattern:     "^(.*)$",
+				Replacement: " (${1})",
+				ResultKey:   "resp_code",
+				Append:      true,
+			},
+			expectedTags: map[string]string{
+				"verb":      "GET",
+				"resp_code": "200 (GET)",
+			},
+		},
 		{
 			message: "Should add new tag",
 			converter: converter{