Fix prometheus_rule_group_last_evaluation_timestamp_seconds

It should be a unix timestamp, not the seconds in the minute.
Signed-off-by: Nbeorn7 <beorn@soundcloud.com>

rules/manager.go

@@ -385,7 +385,7 @@ func (g *Group) GetEvaluationTimestamp() time.Time {
 
 // setEvaluationTimestamp updates evaluationTimestamp to the timestamp of when the rule group was last evaluated.
 func (g *Group) setEvaluationTimestamp(ts time.Time) {
-	g.metrics.groupLastEvalTime.WithLabelValues(groupKey(g.file, g.name)).Set(float64(ts.Second()))
+	g.metrics.groupLastEvalTime.WithLabelValues(groupKey(g.file, g.name)).Set(float64(ts.UnixNano()) / 1e9)
 
 	g.mtx.Lock()
 	defer g.mtx.Unlock()