package main
import (
	"log"
	"net/http"
	"time"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)
func main() {
	var metricSystemTime = prometheus.NewGauge(prometheus.GaugeOpts{
		Name: "custom_system_time", Help: "Timestamp in unix epoch format"})
	prometheus.MustRegister(metricSystemTime)
	go startServer()
	for {
		metricSystemTime.Set(float64(time.Now().Unix()))
		time.Sleep(1 * time.Second)
	}
}
func startServer() {
	http.Handle("/metrics", promhttp.Handler())
	log.Fatal(http.ListenAndServe(":19100", nil))
}

package main
import (
	"log"
	"net/http"
	"time"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)
type Collector struct {
    // Add one of the below lines for each collector you define in the newCollector() function
	systemTime    *prometheus.Desc
}
// Declare your exporter metrics here. Referred to as "collectors"
func newCollector() *Collector {
	return &Collector{
		systemTime: prometheus.NewDesc("my_system_time",
			"System timestamp in unix epoch format",
			nil, nil,
		),
        // Add more collectors (aka metric definitions) here 
	}
}
func (collector *Collector) Describe(ch chan<- *prometheus.Desc) {
	// Add one of these lines for each of your collectors declared above
	ch <- collector.systemTime
}
// This fuction runs when Prometheus scrapes the exporter. It will set a new value for the metric(s)
// I have no idea how it works, but it does.
func (collector *Collector) Collect(ch chan<- prometheus.Metric) {
	var setSystemTime float64
	// Calculate the value of the metric
	setSystemTime = float64(time.Now().Unix())
	// Here is where you set the "new" value for the metric
    // This example is a Gauge, but it can be any Prom metric type
	ch <- prometheus.MustNewConstMetric(collector.systemTime, prometheus.GaugeValue, setSystemTime)
}
// Main function, spawns the collector and the web server
func main() {
	collector := newCollector()
	prometheus.MustRegister(collector)
	http.Handle("/metrics", promhttp.Handler())
	log.Fatal(http.ListenAndServe(":19100", nil))
}