Add maglev_backend_info variable that correlates backend/address/healthcheck/source_tag

internal/checker/checker.go

@@ -378,9 +378,10 @@ func (c *Checker) GetBackendInfo(name string) (metrics.BackendInfo, bool) {
 		return metrics.BackendInfo{}, false
 	}
 	return metrics.BackendInfo{
-		Health:  w.backend,
-		Enabled: w.entry.Enabled,
-		HCName:  w.entry.HealthCheck,
+		Health:    w.backend,
+		Enabled:   w.entry.Enabled,
+		HCName:    w.entry.HealthCheck,
+		SourceTag: w.entry.SourceTag,
 	}, true
 }
 

internal/config/config.go

@@ -123,6 +123,7 @@ type TCPParams struct {
 type Backend struct {
 	Address     net.IP
 	HealthCheck string // name reference into Config.HealthChecks; "" = no probing, assume healthy
+	SourceTag   string // nginx source tag; defaults to the backend name if omitted from config
 	Enabled     bool   // default true; false = exclude from serving entirely
 }
 
@@ -218,7 +219,8 @@ type rawParams struct {
 type rawBackend struct {
 	Address     string `yaml:"address"`
 	HealthCheck string `yaml:"healthcheck"`
-	Enabled     *bool  `yaml:"enabled"` // nil → default true
+	SourceTag   string `yaml:"source-tag"` // defaults to backend name if omitted
+	Enabled     *bool  `yaml:"enabled"`    // nil → default true
 }
 
 type rawPoolBackend struct {
@@ -588,9 +590,14 @@ func convertBackend(name string, r *rawBackend, hcs map[string]HealthCheck) (Bac
 		return Backend{}, fmt.Errorf("invalid address %q", r.Address)
 	}
 
+	sourceTag := r.SourceTag
+	if sourceTag == "" {
+		sourceTag = name
+	}
 	b := Backend{
 		Address:     ip,
 		HealthCheck: r.HealthCheck,
+		SourceTag:   sourceTag,
 		Enabled:     boolDefault(r.Enabled, true),
 	}
 

internal/metrics/metrics.go

@@ -21,9 +21,10 @@ import (
 
 // BackendInfo holds the health and config state needed by the collector.
 type BackendInfo struct {
-	Health  *health.Backend
-	Enabled bool
-	HCName  string // healthcheck name from config
+	Health    *health.Backend
+	Enabled   bool
+	HCName    string // healthcheck name from config
+	SourceTag string // nginx source tag; equals backend name when unset in config
 }
 
 // StateSource provides read-only access to the running checker state.
@@ -134,6 +135,7 @@ type Collector struct {
 	src StateSource
 	vpp VPPSource // optional; nil when VPP integration is disabled
 
+	backendInfo    *prometheus.Desc
 	backendState   *prometheus.Desc
 	backendHealth  *prometheus.Desc
 	backendEnabled *prometheus.Desc
@@ -156,6 +158,11 @@ func NewCollector(src StateSource, vpp VPPSource) *Collector {
 	return &Collector{
 		src: src,
 		vpp: vpp,
+		backendInfo: prometheus.NewDesc(
+			"maglev_backend_info",
+			"Static backend metadata. Always 1; metadata is conveyed via labels.",
+			[]string{"backend", "address", "healthcheck", "source_tag"}, nil,
+		),
 		backendState: prometheus.NewDesc(
 			"maglev_backend_state",
 			"Current backend state (1 = active for the given state label).",
@@ -216,6 +223,7 @@ func NewCollector(src StateSource, vpp VPPSource) *Collector {
 
 // Describe implements prometheus.Collector.
 func (c *Collector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- c.backendInfo
 	ch <- c.backendState
 	ch <- c.backendHealth
 	ch <- c.backendEnabled
@@ -247,6 +255,11 @@ func (c *Collector) Collect(ch chan<- prometheus.Metric) {
 		}
 		addr := info.Health.Address.String()
 
+		ch <- prometheus.MustNewConstMetric(
+			c.backendInfo, prometheus.GaugeValue, 1.0,
+			name, addr, info.HCName, info.SourceTag,
+		)
+
 		// One time-series per possible state; the current state is 1, rest 0.
 		for _, s := range states {
 			val := 0.0