automc ui: per-route active connection counts + total in header

Pulls mc_router_active_connections + mc_router_server_active_connections
from the Prometheus default registry on every /api/routes call. UI
gains an Active column on the routes table and the header subtitle
shows "N routes · M active conn".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/automc/static/index.html

@@ -74,6 +74,7 @@
         <tr>
           <th>Server address (host)</th>
           <th>Backend</th>
+          <th style="text-align:right;">Active</th>
         </tr>
       </thead>
       <tbody id="route-rows"></tbody>
@@ -106,11 +107,14 @@ async function refreshRoutes() {
       count.textContent = '(' + j.routes.length + ')';
       for (const r of j.routes) {
         const tr = document.createElement('tr');
-        tr.innerHTML = '<td>' + r.server_address + '</td><td>' + r.backend + '</td>';
+        tr.innerHTML = '<td>' + r.server_address + '</td>' +
+                       '<td>' + r.backend + '</td>' +
+                       '<td style="text-align:right;font-variant-numeric:tabular-nums;">' + r.active_connections + '</td>';
         rows.appendChild(tr);
       }
     }
-    document.getElementById('meta').textContent = '— ' + j.routes.length + ' routes';
+    document.getElementById('meta').textContent =
+      '— ' + j.routes.length + ' routes · ' + (j.total_connections || 0) + ' active conn';
   } catch (e) {
     document.getElementById('meta').textContent = '— api error';
   }

internal/automc/uipage.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"github.com/itzg/mc-router/server"
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/sirupsen/logrus"
 )
 
@@ -56,26 +57,80 @@ func startUI(ctx context.Context, uiBinding string, bus *LogBus) {
 
 // RouteSnapshot is one row of the routes table the UI renders. Same shape as
 // the upstream /routes JSON but flatter — the UI doesn't need both backend
-// and scalingTarget shown separately.
+// and scalingTarget shown separately. ActiveConnections is the live gauge
+// value scraped from the Prometheus registry (-1 if the metric isn't
+// registered, which can happen briefly at startup).
 type RouteSnapshot struct {
-	ServerAddress string `json:"server_address"`
-	Backend       string `json:"backend"`
+	ServerAddress     string `json:"server_address"`
+	Backend           string `json:"backend"`
+	ActiveConnections int    `json:"active_connections"`
 }
 
 func routesSnapshotHandler(w http.ResponseWriter, _ *http.Request) {
 	mappings := server.Routes.GetMappings()
+	perRoute, total := scrapeActiveConnections()
+
 	out := make([]RouteSnapshot, 0, len(mappings))
 	for addr, backend := range mappings {
-		out = append(out, RouteSnapshot{ServerAddress: addr, Backend: backend})
+		conns, ok := perRoute[addr]
+		if !ok {
+			conns = 0
+		}
+		out = append(out, RouteSnapshot{ServerAddress: addr, Backend: backend, ActiveConnections: conns})
 	}
 	sort.Slice(out, func(i, j int) bool { return out[i].ServerAddress < out[j].ServerAddress })
 	w.Header().Set("Content-Type", "application/json")
 	_ = json.NewEncoder(w).Encode(map[string]any{
-		"routes": out,
-		"at":     time.Now(),
+		"routes":             out,
+		"total_connections":  total,
+		"at":                 time.Now(),
 	})
 }
 
+// scrapeActiveConnections walks the Prometheus default registry to extract
+// two gauges that mc-router exposes:
+//
+//	mc_router_active_connections                          (no labels)
+//	mc_router_server_active_connections{server_address=…} (per route)
+//
+// Returns (perServerAddress, total). On any gathering error returns zero
+// values silently — the UI shows 0 rather than blocking on a metrics issue.
+func scrapeActiveConnections() (map[string]int, int) {
+	per := map[string]int{}
+	total := 0
+	families, err := prometheus.DefaultGatherer.Gather()
+	if err != nil {
+		return per, 0
+	}
+	for _, mf := range families {
+		switch mf.GetName() {
+		case "mc_router_active_connections":
+			for _, m := range mf.GetMetric() {
+				if g := m.GetGauge(); g != nil {
+					total = int(g.GetValue())
+				}
+			}
+		case "mc_router_server_active_connections":
+			for _, m := range mf.GetMetric() {
+				var addr string
+				for _, lp := range m.GetLabel() {
+					if lp.GetName() == "server_address" {
+						addr = lp.GetValue()
+						break
+					}
+				}
+				if addr == "" {
+					continue
+				}
+				if g := m.GetGauge(); g != nil {
+					per[addr] = int(g.GetValue())
+				}
+			}
+		}
+	}
+	return per, total
+}
+
 func sseLogsHandler(bus *LogBus) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/event-stream")