Add default for relabeling

README.adoc

@@ -489,7 +489,7 @@ discussion around the reasoning.
 Dynamic relabeling also allows you to aggregate your metrics by request path (which replaces
 the experimental feature originally introduced in #23). The following example splits the content of
 the `request` variable at every space (using `split`) and return the second element (index 1) of the
-resulting list which is the base for the regex):
+resulting list which is the base for the regex. If nothing was found by regexp, the default value will be returned.:
 
 [source,hcl]
 ----
@@ -506,6 +506,7 @@ namespace "app1" {
 
     match "^/users/[0-9]+" {
       replacement = "/users/:id"
+      default = "/users/unknown"
     }
 
     match "^/profile" {

pkg/config/struct_relabel.go

@@ -24,6 +24,7 @@ type RelabelConfig struct {
 type RelabelValueMatch struct {
 	RegexpString string `hcl:",key" yaml:"regexp"`
 	Replacement  string `hcl:"replacement"`
+	DefaultValue string `hcl:"default"`
 
 	CompiledRegexp *regexp.Regexp
 }

pkg/relabeling/mapping.go

@@ -36,6 +36,8 @@ func (r *Relabeling) Map(sourceValue string) (string, error) {
 			if r.Matches[i].CompiledRegexp.MatchString(sourceValue) {
 				replacement = r.Matches[i].CompiledRegexp.ReplaceAllString(sourceValue, r.Matches[i].Replacement)
 				break
+			} else if r.Matches[i].DefaultValue != "" {
+				replacement = r.Matches[i].DefaultValue
 			}
 		}
 		sourceValue = replacement