Add support for a gauge function and metric to track the maximum number of devices permitted to connect to a Talaria instance.

device/drain/mocks_test.go

@@ -185,3 +185,7 @@ func generateManagerWithDifferentDevices(assert *assert.Assertions, metadataOneC
 
 	return sm
 }
+
+func (sm *stubManager) MaxDevices() int {
+	return 1
+}

device/manager.go

@@ -111,6 +111,7 @@ type Manager interface {
 	Connector
 	Router
 	Registry
+	MaxDevices() int
 }
 
 // ManagerOption is a configuration option for a manager
@@ -630,3 +631,7 @@ func (m *manager) Route(request *Request) (*Response, error) {
 		return nil, ErrorDeviceNotFound
 	}
 }
+
+func (m *manager) MaxDevices() int {
+	return m.devices.limit
+}

xmetrics/registry.go

@@ -38,6 +38,7 @@ type Registry interface {
 
 	NewPrometheusGaugeEx(namespace, subsystem, name string) prometheus.Gauge
 	NewPrometheusGauge(name string) prometheus.Gauge
+	NewGaugeFunc(name string, f func() float64) prometheus.GaugeFunc
 }
 
 // registry is the internal Registry implementation
@@ -128,6 +129,24 @@ func (r *registry) NewGauge(name string) metrics.Gauge {
 	return gokitprometheus.NewGauge(r.NewGaugeVec(name))
 }
 
+func (r *registry) NewGaugeFunc(name string, f func() float64) prometheus.GaugeFunc {
+	gauge := prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Namespace: r.namespace,
+		Subsystem: r.subsystem,
+		Name:      name,
+		Help:      name,
+	}, f)
+
+	if err := r.Register(gauge); err != nil {
+		if already, ok := err.(prometheus.AlreadyRegisteredError); ok {
+			return already.ExistingCollector.(prometheus.GaugeFunc)
+		} else {
+			panic(err)
+		}
+	}
+	return gauge
+}
+
 func (r *registry) NewPrometheusGauge(name string) prometheus.Gauge {
 	return r.NewPrometheusGaugeEx(r.namespace, r.subsystem, name)
 }