feat: allow overwrite the default relabelings.

The "method" label in default relabelings may incorrect if we not use "$request"
in the nginx log_format, but already separated to "$request_method" and
"$request_uri", allow user to overwrite default relabelings to fix it.

main.go

@@ -47,20 +47,31 @@ type Metrics struct {
 	parseErrorsTotal    prometheus.Counter
 }
 
+func inLabels(label string, labels []string) bool {
+	for _, l := range labels {
+		if label == l {
+			return true
+		}
+	}
+	return false
+}
+
 // Init initializes a metrics struct
 func (m *Metrics) Init(cfg *config.NamespaceConfig) {
 	cfg.MustCompile()
 
 	labels := cfg.OrderedLabelNames
 
-	for _, r := range relabeling.DefaultRelabelings {
-		labels = append(labels, r.TargetLabel)
-	}
-
 	for i := range cfg.RelabelConfigs {
 		labels = append(labels, cfg.RelabelConfigs[i].TargetLabel)
 	}
 
+	for _, r := range relabeling.DefaultRelabelings {
+		if !inLabels(r.TargetLabel, labels) {
+			labels = append(labels, r.TargetLabel)
+		}
+	}
+
 	m.countTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: cfg.Name,
 		Name:      "http_response_count_total",
@@ -249,7 +260,8 @@ func processNamespace(nsCfg config.NamespaceConfig) {
 
 func processSourceFile(nsCfg config.NamespaceConfig, t tail.Follower, parser *gonx.Parser, metrics *Metrics) {
 	relabelings := relabeling.NewRelabelings(nsCfg.RelabelConfigs)
-	relabelings = append(relabeling.DefaultRelabelings, relabelings...)
+	relabelings = append(relabelings, relabeling.DefaultRelabelings...)
+	relabelings = relabeling.UniqueRelabelings(relabelings)
 
 	staticLabelValues := nsCfg.OrderedLabelValues
 

relabeling/types.go

@@ -24,3 +24,17 @@ func NewRelabelings(cfgs []config.RelabelConfig) []*Relabeling {
 func NewRelabeling(cfg *config.RelabelConfig) *Relabeling {
 	return &Relabeling{*cfg}
 }
+
+// UniqueRelabelings creates a unique relabelings, the duplicated one at the end will discard.
+func UniqueRelabelings(relabelings []*Relabeling) []*Relabeling {
+	result := make([]*Relabeling, 0, len(relabelings))
+	found := make(map[string]struct{})
+	for _, r := range relabelings {
+		if _, ok := found[r.TargetLabel]; ok {
+			continue
+		}
+		found[r.TargetLabel] = struct{}{}
+		result = append(result, r)
+	}
+	return result
+}