store/proxy: set {Min,Max}Time accordingly to the nodes (#982)

* store/proxy: set {Min,Max}Time accordingly to the nodes

It doesn't make sense to advertise that a Thanos Query node has data
since the beginning of the time and till the end of time if it only has
nodes which have specific time ranges of data.

Change it accordingly by checking each time range of each node and
setting it to the min/max values.

* store/proxy: add handling for edge case

In case there are no configured stores at Thanos Query we want to revert
to the old behaviour and inform the Info() callers that indeed we do
have all of the data.

pkg/store/proxy.go

@@ -75,9 +75,31 @@ func (s *ProxyStore) Info(ctx context.Context, r *storepb.InfoRequest) (*storepb
 	res := &storepb.InfoResponse{
 		Labels:    make([]storepb.Label, 0, len(s.selectorLabels)),
 		StoreType: s.component.ToProto(),
-		MinTime:   0,
-		MaxTime:   math.MaxInt64,
 	}
+
+	MinTime := int64(math.MaxInt64)
+	MaxTime := int64(0)
+
+	stores := s.stores()
+	for _, s := range stores {
+		mint, maxt := s.TimeRange()
+		if mint < MinTime {
+			MinTime = mint
+		}
+		if maxt > MaxTime {
+			MaxTime = maxt
+		}
+	}
+
+	// Edge case: we have all of the data if there are no stores.
+	if len(stores) == 0 {
+		MinTime = 0
+		MaxTime = math.MaxInt64
+	}
+
+	res.MaxTime = MaxTime
+	res.MinTime = MinTime
+
 	for _, l := range s.selectorLabels {
 		res.Labels = append(res.Labels, storepb.Label{
 			Name:  l.Name,
@@ -393,7 +415,7 @@ func (s *ProxyStore) LabelValues(ctx context.Context, r *storepb.LabelValuesRequ
 		store := st
 		g.Go(func() error {
 			resp, err := store.LabelValues(gctx, &storepb.LabelValuesRequest{
-				Label:                   r.Label,
+				Label: r.Label,
 				PartialResponseDisabled: r.PartialResponseDisabled,
 			})
 			if err != nil {