Add prometheus metrics exporter

This should add metrics parity to the Python version

metrics.go

@@ -0,0 +1,101 @@
+package main
+
+import (
+	"fmt"
+	"github.com/prometheus/client_golang/prometheus"
+	// "github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"net/http"
+)
+
+// TODO: Not sure if this is the best way to handle. A global instance for
+// metrics isn't bad, but it might be nice to curry versions of the metrics
+// for each monitor. Especially since every monitor has it's own. Perhaps
+// another new function that essentially curries each metric for a given
+// monitor name would do. This could be run when validating monitors and
+// initializing alert templates.
+
+// MinitorMetrics contains all counters and metrics that Minitor will need to access
+type MinitorMetrics struct {
+	alertCount    *prometheus.CounterVec
+	checkCount    *prometheus.CounterVec
+	monitorStatus *prometheus.GaugeVec
+}
+
+// NewMetrics creates and initializes all metrics
+func NewMetrics() *MinitorMetrics {
+	// Initialize all metrics
+	metrics := &MinitorMetrics{
+		alertCount: prometheus.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "minitor_alert_total",
+				Help: "Number of Minitor alerts",
+			},
+			[]string{"alert", "monitor"},
+		),
+		checkCount: prometheus.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "minitor_check_total",
+				Help: "Number of Minitor checks",
+			},
+			[]string{"monitor", "status", "is_alert"},
+		),
+		monitorStatus: prometheus.NewGaugeVec(
+			prometheus.GaugeOpts{
+				Name: "minitor_monitor_up_count",
+				Help: "Status of currently responsive monitors",
+			},
+			[]string{"monitor"},
+		),
+	}
+
+	// Register newly created metrics
+	prometheus.MustRegister(metrics.alertCount)
+	prometheus.MustRegister(metrics.checkCount)
+	prometheus.MustRegister(metrics.monitorStatus)
+
+	return metrics
+}
+
+// SetMonitorStatus sets the current status of Monitor
+func (metrics *MinitorMetrics) SetMonitorStatus(monitor string, isUp bool) {
+	val := 0.0
+	if isUp {
+		val = 1.0
+	}
+	metrics.monitorStatus.With(prometheus.Labels{"monitor": monitor}).Set(val)
+}
+
+// CountCheck counts the result of a particular Monitor check
+func (metrics *MinitorMetrics) CountCheck(monitor string, isSuccess bool, isAlert bool) {
+	status := "failure"
+	if isSuccess {
+		status = "success"
+	}
+
+	alertVal := "false"
+	if isAlert {
+		alertVal = "true"
+	}
+
+	metrics.checkCount.With(
+		prometheus.Labels{"monitor": monitor, "status": status, "is_alert": alertVal},
+	).Inc()
+}
+
+// CountAlert counts an alert
+func (metrics *MinitorMetrics) CountAlert(monitor string, alert string) {
+	metrics.alertCount.With(
+		prometheus.Labels{
+			"alert":   alert,
+			"monitor": monitor,
+		},
+	).Inc()
+}
+
+// ServeMetrics starts an http server with a Prometheus metrics handler
+func ServeMetrics() {
+	http.Handle("/metrics", promhttp.Handler())
+	host := fmt.Sprintf(":%d", MetricsPort)
+	_ = http.ListenAndServe(host, nil)
+}