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

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),
 	}