feat: expose timestamp for when current metrics are from

Since metrics are not live, but an arbitrary date in the past few days
depending on when reports are available to, it seems like a reasonable
idea to include the timestamp of the date that the current data is from.

main.go

@@ -26,13 +26,18 @@ import (
 var statsTemplate string
 
 type QuotaCollector struct {
-	total  *prometheus.Desc
-	used   *prometheus.Desc
-	client *admin.Service
+	timestamp *prometheus.Desc
+	total     *prometheus.Desc
+	used      *prometheus.Desc
+	client    *admin.Service
 }
 
 func NewQuotaCollector(client *admin.Service) *QuotaCollector {
 	return &QuotaCollector{
+		timestamp: prometheus.NewDesc("google_workspace_quota_timestamp",
+			"Timestamp of the quota stats",
+			nil, nil,
+		),
 		total: prometheus.NewDesc("google_workspace_quota_bytes_total",
 			"Total quota in bytes",
 			nil, nil,
@@ -51,7 +56,7 @@ func (c *QuotaCollector) Describe(ch chan<- *prometheus.Desc) {
 }
 
 func (c *QuotaCollector) Collect(ch chan<- prometheus.Metric) {
-	_, totalQuota, usedQuota, _, err := c.fetchQuotaStats()
+	t, totalQuota, usedQuota, _, err := c.fetchQuotaStats()
 	if err != nil {
 		slog.Error(
 			"Failed to fetch quota stats",
@@ -60,6 +65,9 @@ func (c *QuotaCollector) Collect(ch chan<- prometheus.Metric) {
 		return
 	}
 
+	ch <- prometheus.MustNewConstMetric(
+		c.timestamp, prometheus.GaugeValue, float64(t.Unix()),
+	)
 	ch <- prometheus.MustNewConstMetric(
 		c.total, prometheus.GaugeValue, totalQuota*1048576,
 	)