Add scrape target path and scheme relabelling

cmd/promxy/config.yaml

@@ -45,6 +45,7 @@ promxy:
       # configures the path to send remote read requests to. The default is "api/v1/read"
       remote_read_path: api/v1/read
       # path_prefix defines a prefix to prepend to all queries to hosts in this servergroup
+      # This can be relabeled using __path_prefix__
       path_prefix: /example/prefix
       # query_params adds the following map of query parameters to downstream requests.
       # The initial use-case for this is to add `nocache=1` to VictoriaMetrics downstreams

pkg/servergroup/config.go

@@ -23,6 +23,11 @@ var (
 	}
 )
 
+const (
+	// PathPrefixLabel is the name of the label that holds the path prefix for a scrape target.
+	PathPrefixLabel = "__path_prefix__"
+)
+
 // Config is the configuration for a ServerGroup that promxy will talk to.
 // This is where the vast majority of options exist.
 type Config struct {

pkg/servergroup/servergroup.go

@@ -109,7 +109,8 @@ SYNC_LOOP:
 		for _, targetGroupList := range targetGroupMap {
 			for _, targetGroup := range targetGroupList {
 				for _, target := range targetGroup.Targets {
-					lbls := make([]labels.Label, 0, len(target)+len(targetGroup.Labels))
+
+					lbls := make([]labels.Label, 0, len(target)+len(targetGroup.Labels)+2)
 
 					for ln, lv := range target {
 						lbls = append(lbls, labels.Label{Name: string(ln), Value: string(lv)})
@@ -121,7 +122,11 @@ SYNC_LOOP:
 						}
 					}
 
+					lbls = append(lbls, labels.Label{Name: model.SchemeLabel, Value: string(s.Cfg.Scheme)})
+					lbls = append(lbls, labels.Label{Name: PathPrefixLabel, Value: string(s.Cfg.PathPrefix)})
+
 					lset := labels.New(lbls...)
+
 					logrus.Tracef("Potential target pre-relabel: %v", lset)
 					lset = relabel.Process(lset, s.Cfg.RelabelConfigs...)
 					logrus.Tracef("Potential target post-relabel: %v", lset)
@@ -137,10 +142,11 @@ SYNC_LOOP:
 					}
 
 					u := &url.URL{
-						Scheme: string(s.Cfg.GetScheme()),
-						Host:   string(lset.Get(model.AddressLabel)),
-						Path:   s.Cfg.PathPrefix,
+						Scheme: lset.Get(model.SchemeLabel),
+						Host:   lset.Get(model.AddressLabel),
+						Path:   lset.Get(PathPrefixLabel),
 					}
+
 					targets = append(targets, u.Host)
 
 					client, err := api.NewClient(api.Config{Address: u.String(), RoundTripper: s.client.Transport})