ipn/ipnlocal, health: add home DERP region usermetric

Fixes #18061

Signed-off-by: Raj Singh <raj@tailscale.com>

health/health.go

@@ -761,6 +761,16 @@ func (t *Tracker) SetMagicSockDERPHome(region int, homeless bool) {
 	t.selfCheckLocked()
 }
 
+// DERPHomeRegion returns the current home DERP region ID, or 0 if unknown.
+func (t *Tracker) DERPHomeRegion() int {
+	if t.nil() {
+		return 0
+	}
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.derpHomeRegion
+}
+
 // NoteMapRequestHeard notes whenever we successfully sent a map request
 // to control for which we received a 200 response.
 func (t *Tracker) NoteMapRequestHeard(mr *tailcfg.MapRequest) {

ipn/ipnlocal/local.go

@@ -439,6 +439,8 @@ type metrics struct {
 	// approvedRoutes is a metric that reports the number of network routes served by the local node and approved
 	// by the control server.
 	approvedRoutes *usermetric.Gauge
+
+	homeDERP *usermetric.Gauge
 }
 
 // clientGen is a func that creates a control plane client.
@@ -490,6 +492,8 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 			"tailscaled_advertised_routes", "Number of advertised network routes (e.g. by a subnet router)"),
 		approvedRoutes: sys.UserMetricsRegistry().NewGauge(
 			"tailscaled_approved_routes", "Number of approved network routes (e.g. by a subnet router)"),
+		homeDERP: sys.UserMetricsRegistry().NewGauge(
+			"tailscaled_home_derp_region_id", "DERP region ID of this node's home relay server"),
 	}
 
 	b := &LocalBackend{
@@ -6153,6 +6157,8 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 		}
 	}
 
+	b.metrics.homeDERP.Set(float64(b.health.DERPHomeRegion()))
+
 	if buildfeatures.HasDrive && nm != nil {
 		if f, ok := hookSetNetMapLockedDrive.GetOk(); ok {
 			f(b, nm)