NE-2418: Add haproxy_max_connections metric

Add a new haproxy_max_connections gauge metric that exposes the
process-wide maximum connections configured for HAProxy.

The metric is extracted from the public frontend's "slim" field
(field 6) in HAProxy's "show stat" CSV output. Since the router
configures both global and defaults sections with the same
ROUTER_MAX_CONNECTIONS value, the public frontend's session limit
reflects the process-wide maxconn setting.

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

Author: alebedev87

pkg/router/metrics/haproxy/haproxy.go

@@ -136,6 +136,7 @@ type Exporter struct {
 	up, nextScrapeInterval                         prometheus.Gauge
 	totalScrapes, csvParseFailures                 prometheus.Counter
 	serverThresholdCurrent, serverThresholdLimit   prometheus.Gauge
+	maxConnections                                 prometheus.Gauge
 	frontendMetrics, backendMetrics, serverMetrics map[int]*prometheus.GaugeVec
 
 	// counterValues is added to the value specific haproxy frontend, backend, or server counter
@@ -218,6 +219,11 @@ func NewExporter(opts PrometheusOptions) (*Exporter, error) {
 			Name:      "exporter_csv_parse_failures",
 			Help:      "Number of errors while parsing CSV.",
 		}),
+		maxConnections: prometheus.NewGauge(prometheus.GaugeOpts{
+			Namespace: namespace,
+			Name:      "max_connections",
+			Help:      "Maximum number of concurrent connections configured for the HAProxy process.",
+		}),
 		frontendMetrics: filterMetrics(opts.ExportedMetrics, metrics{
 			4:  newFrontendMetric("current_sessions", "Current number of active sessions.", nil),
 			5:  newFrontendMetric("max_sessions", "Maximum observed number of active sessions.", nil),
@@ -321,6 +327,7 @@ func (e *Exporter) Describe(ch chan<- *prometheus.Desc) {
 	ch <- e.serverThresholdCurrent.Desc()
 	ch <- e.serverThresholdLimit.Desc()
 	ch <- e.csvParseFailures.Desc()
+	ch <- e.maxConnections.Desc()
 }
 
 // Collect fetches the stats from configured HAProxy location and delivers them
@@ -347,6 +354,7 @@ func (e *Exporter) Collect(ch chan<- prometheus.Metric) {
 	ch <- e.serverThresholdCurrent
 	ch <- e.serverThresholdLimit
 	ch <- e.csvParseFailures
+	ch <- e.maxConnections
 	e.collectMetrics(ch)
 }
 
@@ -516,6 +524,19 @@ func (e *Exporter) parseRow(csvRow []string, updatedValues counterValuesByMetric
 
 	switch typ {
 	case frontendType:
+		// Extract maxconn from public frontend for the process-wide metric.
+		// Field 6 corresponds to "slim" (session limit) column from HAProxy's "show stat" CSV output.
+		// The router configures both global and defaults sections with the same ROUTER_MAX_CONNECTIONS value,
+		// so the public frontend's limit (field 6/slim) reflects the process-wide maxconn setting.
+		// NOTE: If the defaults maxconn is ever configured differently from global maxconn,
+		// this approach will no longer accurately represent the process-wide limit.
+		if pxname == "public" && len(csvRow) > 6 {
+			if maxConnStr := csvRow[6]; maxConnStr != "" {
+				if maxConn, err := strconv.ParseInt(maxConnStr, 10, 64); err == nil {
+					e.maxConnections.Set(float64(maxConn))
+				}
+			}
+		}
 		e.exportAndRecordRow(e.frontendMetrics, metricID{proxyType: serverType, proxyName: pxname}, updatedValues, csvRow, pxname)
 	case backendType:
 		if mode, value, ok := knownBackendSegment(pxname); ok {