Persist time series viewer metrics (#1752)

* WIP persisting time series viewer metrics. Closes #1478

* Remove unused var, move around initial collection

metrics/timestampedValue.go

@@ -1,8 +1,21 @@
 package metrics
 
-import "time"
+import (
+	"time"
+
+	"github.com/nakabonne/tstorage"
+)
 
 type timestampedValue struct {
 	Time  time.Time `json:"time"`
 	Value int       `json:"value"`
 }
+
+func makeTimestampedValuesFromDatapoints(dp []*tstorage.DataPoint) []timestampedValue {
+	tv := []timestampedValue{}
+	for _, d := range dp {
+		tv = append(tv, timestampedValue{Time: time.Unix(d.Timestamp, 0), Value: int(d.Value)})
+	}
+
+	return tv
+}