Populate num_user field for VPN subsystem metrics

Fixes #417

UniFi controllers populate RemoteUserNumActive for VPN connections but
leave NumUser at 0 for the VPN subsystem. This caused dashboard queries
looking for num_user in the VPN subsystem to always show 0 active users,
even when VPN connections were active.

Root Cause:
For most subsystems (wlan, lan, www), the controller populates NumUser
directly. However, for the VPN subsystem, the controller uses the
RemoteUserNumActive field instead, leaving NumUser at 0.

The Prometheus exporter had special handling for VPN (lines 148-156 in
pkg/promunifi/site.go) and exported RemoteUserNumActive, but did not
export NumUser. The InfluxDB and Datadog exporters exported all fields
for all subsystems without special handling, resulting in num_user
always being 0 for VPN.

Existing Grafana dashboards query:
  SELECT "num_user" FROM "subsystems" WHERE subsystem='vpn'

This always returned 0 even with active VPN users.

Solution:
For all three exporters (InfluxDB, Datadog, Prometheus), when the
subsystem is 'vpn' and NumUser is 0 but RemoteUserNumActive has a
value, populate num_user with RemoteUserNumActive.

Changes:
- pkg/influxunifi/site.go: Add VPN-specific num_user fallback logic
- pkg/datadogunifi/site.go: Add VPN-specific num_user fallback logic
- pkg/promunifi/site.go: Add NumUser metric to VPN case with fallback

This maintains backward compatibility - existing queries for num_user
will now work correctly, and the remote_user_num_active field is still
available for those who updated their dashboards.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

pkg/datadogunifi/site.go

@@ -22,8 +22,14 @@ func (u *DatadogUnifi) reportSite(r report, s *unifi.Site) {
 			tag("lan_ip", h.LanIP),
 		}
 
+		// For VPN subsystem, use RemoteUserNumActive for num_user if NumUser is not set
+		numUser := h.NumUser.Val
+		if h.Subsystem == "vpn" && numUser == 0 && h.RemoteUserNumActive.Val > 0 {
+			numUser = h.RemoteUserNumActive.Val
+		}
+
 		data := map[string]float64{
-			"num_user":                 h.NumUser.Val,
+			"num_user":                 numUser,
 			"num_guest":                h.NumGuest.Val,
 			"num_iot":                  h.NumIot.Val,
 			"tx_bytes_r":               h.TxBytesR.Val,

pkg/influxunifi/site.go

@@ -19,8 +19,15 @@ func (u *InfluxUnifi) batchSite(r report, s *unifi.Site) {
 			"gw_name":   h.GwName,
 			"lan_ip":    h.LanIP,
 		}
+
+		// For VPN subsystem, use RemoteUserNumActive for num_user if NumUser is not set
+		numUser := h.NumUser.Val
+		if h.Subsystem == "vpn" && numUser == 0 && h.RemoteUserNumActive.Val > 0 {
+			numUser = h.RemoteUserNumActive.Val
+		}
+
 		fields := map[string]any{
-			"num_user":                 h.NumUser.Val,
+			"num_user":                 numUser,
 			"num_guest":                h.NumGuest.Val,
 			"num_iot":                  h.NumIot.Val,
 			"tx_bytes-r":               h.TxBytesR.Int64(),

pkg/promunifi/site.go

@@ -146,7 +146,14 @@ func (u *promUnifi) exportSite(r report, s *unifi.Site) {
 				{u.Site.NumSw, gauge, h.NumSw, labels},
 			})
 		case "vpn":
+			// For VPN subsystem, use RemoteUserNumActive for NumUser if NumUser is not set
+			numUser := h.NumUser.Val
+			if numUser == 0 && h.RemoteUserNumActive.Val > 0 {
+				numUser = h.RemoteUserNumActive.Val
+			}
+
 			r.send([]*metric{
+				{u.Site.NumUser, gauge, numUser, labels},
 				{u.Site.RemoteUserNumActive, gauge, h.RemoteUserNumActive, labels},
 				{u.Site.RemoteUserNumInactive, gauge, h.RemoteUserNumInactive, labels},
 				{u.Site.RemoteUserRxBytes, counter, h.RemoteUserRxBytes, labels},