stats: add primary info metric

Signed-off-by: Janusz Marcinkiewicz <januszm@nvidia.com>

Author: VirrageS

ais/proxy.go

@@ -211,6 +211,7 @@ func (p *proxy) Run() error {
 
 	p.notifs.init(p)
 	p.ic.init(p)
+	stats.RegSmapMetrics(p.owner.smap)
 
 	p.initRecvHandlers()
 

ais/target.go

@@ -480,6 +480,7 @@ func (t *target) Run() error {
 	tstats.RegMetrics(t.si)
 
 	t.initBackends() // (+ reg backend metrics)
+	stats.RegSmapMetrics(t.owner.smap)
 	// end target metrics -----------------------
 
 	// probe xattrs

stats/primary.go

@@ -0,0 +1,53 @@
+// Package stats provides methods and functionality to register, track, log,
+// and export metrics that, for the most part, include "counter" and "latency" kinds.
+/*
+ * Copyright (c) 2026, NVIDIA CORPORATION. All rights reserved.
+ */
+package stats
+
+import (
+	"github.com/NVIDIA/aistore/core/meta"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const primaryIDLabel = "primary_id"
+
+type primaryInfo struct {
+	sowner meta.Sowner
+	desc   *prometheus.Desc
+}
+
+func RegSmapMetrics(sowner meta.Sowner) {
+	promRegistry.MustRegister(newPrimaryInfo(sowner))
+}
+
+func newPrimaryInfo(sowner meta.Sowner) *primaryInfo {
+	return &primaryInfo{
+		sowner: sowner,
+		desc: prometheus.NewDesc(
+			prometheus.BuildFQName("ais", "node", "primary_info"),
+			"Primary proxy information for this node.",
+			[]string{primaryIDLabel},
+			prometheus.Labels{ConstlabNode: staticLabs[ConstlabNode]},
+		),
+	}
+}
+
+func (p *primaryInfo) Describe(ch chan<- *prometheus.Desc) {
+	ch <- p.desc
+}
+
+func (p *primaryInfo) Collect(ch chan<- prometheus.Metric) {
+	smap := p.sowner.Get()
+	if smap == nil || smap.Primary == nil {
+		return
+	}
+
+	ch <- prometheus.MustNewConstMetric(
+		p.desc,
+		prometheus.GaugeValue,
+		1,
+		smap.Primary.ID(),
+	)
+}

stats/primary_internal_test.go

@@ -0,0 +1,107 @@
+// Package stats provides methods and functionality to register, track, log,
+// and export metrics that, for the most part, include "counter" and "latency" kinds.
+/*
+ * Copyright (c) 2026, NVIDIA CORPORATION. All rights reserved.
+ */
+package stats
+
+import (
+	"testing"
+
+	"github.com/NVIDIA/aistore/api/apc"
+	"github.com/NVIDIA/aistore/core/meta"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func TestPrimaryInfoMetricTracksSmapPrimary(t *testing.T) {
+	prevNodeID, hadNodeID := staticLabs[ConstlabNode]
+	t.Cleanup(func() {
+		if hadNodeID {
+			staticLabs[ConstlabNode] = prevNodeID
+		} else {
+			delete(staticLabs, ConstlabNode)
+		}
+	})
+	staticLabs[ConstlabNode] = "node1"
+	sowner := &testSowner{smap: primarySmap("primary1")}
+
+	reg := prometheus.NewRegistry()
+	reg.MustRegister(newPrimaryInfo(sowner))
+
+	val, nodeID, primaryID := gatherPrimaryInfo(t, reg)
+	if val != 1 {
+		t.Fatalf("expected primary1 metric value 1, got %v", val)
+	}
+	if nodeID != staticLabs[ConstlabNode] {
+		t.Fatalf("expected node_id label %q, got %q", staticLabs[ConstlabNode], nodeID)
+	}
+	if primaryID != "primary1" {
+		t.Fatalf("expected primary_id label primary1, got %q", primaryID)
+	}
+
+	sowner.smap = primarySmap("primary2")
+	val, nodeID, primaryID = gatherPrimaryInfo(t, reg)
+	if val != 1 {
+		t.Fatalf("expected primary2 metric value 1, got %v", val)
+	}
+	if nodeID != staticLabs[ConstlabNode] {
+		t.Fatalf("expected node_id label %q, got %q", staticLabs[ConstlabNode], nodeID)
+	}
+	if primaryID != "primary2" {
+		t.Fatalf("expected primary_id label primary2, got %q", primaryID)
+	}
+}
+
+func gatherPrimaryInfo(t *testing.T, reg *prometheus.Registry) (val float64, nodeID, primaryID string) {
+	t.Helper()
+
+	families, err := reg.Gather()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, fam := range families {
+		if fam.GetName() != "ais_node_primary_info" {
+			continue
+		}
+		ms := fam.GetMetric()
+		if len(ms) != 1 {
+			t.Fatalf("expected 1 sample, got %d", len(ms))
+		}
+		m := ms[0]
+		g := m.GetGauge()
+		if g == nil {
+			t.Fatal("expected gauge metric")
+		}
+		for _, lp := range m.GetLabel() {
+			switch lp.GetName() {
+			case ConstlabNode:
+				nodeID = lp.GetValue()
+			case primaryIDLabel:
+				primaryID = lp.GetValue()
+			}
+		}
+		if nodeID == "" {
+			t.Fatal("expected node_id label")
+		}
+		if primaryID == "" {
+			t.Fatal("expected primary_id label")
+		}
+		return g.GetValue(), nodeID, primaryID
+	}
+	t.Fatal("metric ais_node_primary_info not found")
+	return 0, "", ""
+}
+
+func primarySmap(primaryID string) *meta.Smap {
+	primary := &meta.Snode{}
+	primary.Init(primaryID, apc.Proxy)
+	return &meta.Smap{Primary: primary}
+}
+
+type testSowner struct {
+	smap *meta.Smap
+}
+
+func (s *testSowner) Get() *meta.Smap             { return s.smap }
+func (*testSowner) Listeners() meta.SmapListeners { return nil }