[#77] Add metrics for HTTP endpoint status

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

metrics/desc.go

@@ -66,6 +66,16 @@ var appMetricsDesc = map[string]map[string]Description{
 			VariableLabels: []string{"version"},
 		},
 	},
+	serverSubsystem: {
+		healthMetric: Description{
+			Type:           dto.MetricType_GAUGE,
+			Namespace:      namespace,
+			Subsystem:      serverSubsystem,
+			Name:           healthMetric,
+			Help:           "HTTP Server endpoint health",
+			VariableLabels: []string{"endpoint"},
+		},
+	},
 }
 
 type Description struct {