feat(metrics): add Prometheus /metrics endpoint

Zero-dependency Prometheus text exposition format. Exposes monitor
up/down, latency, status code, check timestamps, pause state,
SSL cert expiry, and check counters — all from in-memory state.

internal/metrics/prometheus.go

@@ -0,0 +1,99 @@
+package metrics
+
+import (
+	"fmt"
+	"go-upkeep/internal/models"
+	"go-upkeep/internal/monitor"
+	"net/http"
+	"sort"
+	"strings"
+)
+
+func Handler(eng *monitor.Engine) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		sites := eng.GetAllSites()
+		sort.Slice(sites, func(i, j int) bool { return sites[i].ID < sites[j].ID })
+
+		var b strings.Builder
+
+		writeHelp(&b, "upkeep_monitor_up", "gauge", "Whether the monitor is up (1) or down (0).")
+		for _, s := range sites {
+			val := 0
+			if s.Status == "UP" {
+				val = 1
+			}
+			writeGauge(&b, "upkeep_monitor_up", labels(s), float64(val))
+		}
+
+		writeHelp(&b, "upkeep_monitor_latency_seconds", "gauge", "Last check latency in seconds.")
+		for _, s := range sites {
+			writeGauge(&b, "upkeep_monitor_latency_seconds", labels(s), s.Latency.Seconds())
+		}
+
+		writeHelp(&b, "upkeep_monitor_status_code", "gauge", "HTTP response status code of the last check.")
+		for _, s := range sites {
+			if s.Type != "http" {
+				continue
+			}
+			writeGauge(&b, "upkeep_monitor_status_code", labels(s), float64(s.StatusCode))
+		}
+
+		writeHelp(&b, "upkeep_monitor_check_timestamp_seconds", "gauge", "Unix timestamp of the last check.")
+		for _, s := range sites {
+			if s.LastCheck.IsZero() {
+				continue
+			}
+			writeGauge(&b, "upkeep_monitor_check_timestamp_seconds", labels(s), float64(s.LastCheck.Unix()))
+		}
+
+		writeHelp(&b, "upkeep_monitor_paused", "gauge", "Whether the monitor is paused (1) or active (0).")
+		for _, s := range sites {
+			val := 0
+			if s.Paused {
+				val = 1
+			}
+			writeGauge(&b, "upkeep_monitor_paused", labels(s), float64(val))
+		}
+
+		writeHelp(&b, "upkeep_monitor_cert_expiry_timestamp_seconds", "gauge", "Unix timestamp when the SSL certificate expires.")
+		for _, s := range sites {
+			if !s.HasSSL || s.CertExpiry.IsZero() {
+				continue
+			}
+			writeGauge(&b, "upkeep_monitor_cert_expiry_timestamp_seconds", labels(s), float64(s.CertExpiry.Unix()))
+		}
+
+		writeHelp(&b, "upkeep_monitor_checks_total", "counter", "Total number of checks performed.")
+		writeHelp(&b, "upkeep_monitor_checks_up_total", "counter", "Total number of successful checks.")
+		for _, s := range sites {
+			h, ok := eng.GetHistory(s.ID)
+			if !ok {
+				continue
+			}
+			writeGauge(&b, "upkeep_monitor_checks_total", labels(s), float64(h.TotalChecks))
+			writeGauge(&b, "upkeep_monitor_checks_up_total", labels(s), float64(h.UpChecks))
+		}
+
+		w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
+		w.Write([]byte(b.String()))
+	}
+}
+
+func labels(s models.Site) string {
+	return fmt.Sprintf(`id="%d",name="%s",type="%s"`, s.ID, escapeLabelValue(s.Name), s.Type)
+}
+
+func escapeLabelValue(s string) string {
+	s = strings.ReplaceAll(s, `\`, `\\`)
+	s = strings.ReplaceAll(s, `"`, `\"`)
+	s = strings.ReplaceAll(s, "\n", `\n`)
+	return s
+}
+
+func writeHelp(b *strings.Builder, name, typ, help string) {
+	fmt.Fprintf(b, "# HELP %s %s\n# TYPE %s %s\n", name, help, name, typ)
+}
+
+func writeGauge(b *strings.Builder, name, labels string, val float64) {
+	fmt.Fprintf(b, "%s{%s} %g\n", name, labels, val)
+}