telemetry: fix total disk usage over time (#10476)

* telemetry: total disk usage over time counted each cluster on one day

GetMetrics aggregated s.instances, which holds only each cluster's most
recent report. Every cluster therefore landed in a single date bucket --
the day it last reported on -- so the chart plotted the disk usage of
clusters that went silent that day, and piled the whole live fleet onto
today. Aggregate the daily histories instead, reusing the day alignment
that the per-cluster size series already does.

* telemetry: don't pad the charts with days the server has no history for

The dashboard asks for 30 days, but daily history only starts when a
server first collects it, so the charts opened on a run of zeros and then
jumped -- reading as a fleet that appeared overnight. Start the window at
the oldest sample on hand when it is younger than the requested range.

telemetry/server/storage/sizes_test.go

@@ -10,13 +10,17 @@ import (
 
 // seedHistory gives a cluster one sample per listed day offset (0 is today).
 func seedHistory(s *PrometheusStorage, id string, disk uint64, dayOffsets ...int) {
+	seedSamples(s, id, HistorySample{TotalDiskBytes: disk}, dayOffsets...)
+}
+
+// seedSamples is seedHistory for tests that care about more than disk usage.
+// The sample's Ts is filled in per day offset.
+func seedSamples(s *PrometheusStorage, id string, sample HistorySample, dayOffsets ...int) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	for _, offset := range dayOffsets {
-		s.histories[id] = append(s.histories[id], HistorySample{
-			Ts:             time.Now().AddDate(0, 0, offset).Unix(),
-			TotalDiskBytes: disk,
-		})
+		sample.Ts = time.Now().AddDate(0, 0, offset).Unix()
+		s.histories[id] = append(s.histories[id], sample)
 	}
 }
 
@@ -97,11 +101,16 @@ func TestClusterSizeSeriesUsesLatestDailySample(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// Nothing was reported before today, so the axis is today alone rather
+	// than the full 3 days padded out with zeros.
 	series := s.GetClusterSizeSeries(3, 0)
 	if len(series.Clusters) != 1 {
 		t.Fatalf("clusters = %+v, want 1", series.Clusters)
 	}
-	if got := series.Clusters[0].Disk; !equal(got, []uint64{0, 0, 700}) {
+	if today := time.Now().UTC().Format("2006-01-02"); len(series.Dates) != 1 || series.Dates[0] != today {
+		t.Errorf("dates = %v, want %s only", series.Dates, today)
+	}
+	if got := series.Clusters[0].Disk; !equal(got, []uint64{700}) {
 		t.Errorf("disk = %v, want today's latest sample only", got)
 	}
 	if series.TotalDisk != 700 {