Add health check and self reporting of health

This avoids panicing and instead provides an HTTP endpoint to report health

health.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+)
+
+type HealthCheckHandler struct {
+	isMinitorHealthy bool
+	monitors         []*Monitor
+}
+
+func NewHealthCheckHandler(monitors []*Monitor) *HealthCheckHandler {
+	return &HealthCheckHandler{
+		false,
+		monitors,
+	}
+}
+
+func (hch *HealthCheckHandler) MinitorHealthy(healthy bool) {
+	hch.isMinitorHealthy = healthy
+}
+
+func (hch HealthCheckHandler) MinitorHealthCheck() (bool, string) {
+	if hch.isMinitorHealthy {
+		return true, "OK"
+	} else {
+		return false, "UNHEALTHY"
+	}
+}
+
+func (hch HealthCheckHandler) MonitorsHealthCheck() (bool, string) {
+	downMonitors := []string{}
+
+	for _, monitor := range hch.monitors {
+		if !monitor.IsUp() {
+			downMonitors = append(downMonitors, monitor.Name)
+		}
+	}
+
+	if len(downMonitors) == 0 {
+		return true, "OK"
+	} else {
+		return false, fmt.Sprintf("UNHEALTHY: The following monitors are unhealthy: %s", strings.Join(downMonitors, ", "))
+	}
+}
+
+func (hch HealthCheckHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	var healthy bool
+
+	var body string
+
+	if monitors := r.URL.Query().Get("monitors"); monitors != "" {
+		healthy, body = hch.MonitorsHealthCheck()
+	} else {
+		healthy, body = hch.MinitorHealthCheck()
+	}
+
+	if healthy {
+		w.WriteHeader(http.StatusOK)
+	} else {
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}
+
+	_, _ = io.WriteString(w, body)
+}
+
+func HandleHealthCheck() {
+	http.Handle("/metrics", HealthChecks)
+}