Add staggering between identical start and end times (#321)

* Add staggering between identical start and end times

metrics.go

diff --git a/metrics.go b/metrics.go
index 1516c35..fee316d 100644
--- a/metrics.go
+++ b/metrics.go
@@ -411,12 +411,16 @@
 		return nil, err
 	}
 
+	interval := &monitoringpb.TimeInterval{
+		StartTime: startTime,
+		EndTime:   timestampProto(pt.Time),
+	}
+	if startTime != nil {
+		interval = toValidTimeIntervalpb(startTime.AsTime(), pt.Time)
+	}
 	mpt := &monitoringpb.Point{
-		Value: mptv,
-		Interval: &monitoringpb.TimeInterval{
-			StartTime: startTime,
-			EndTime:   timestampProto(pt.Time),
-		},
+		Value:    mptv,
+		Interval: interval,
 	}
 	return mpt, nil
 }

metrics_test.go

diff --git a/metrics_test.go b/metrics_test.go
index 50be3a9..cf7d953 100644
--- a/metrics_test.go
+++ b/metrics_test.go
@@ -19,6 +19,7 @@
 	"fmt"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/golang/protobuf/ptypes/any"
 	"github.com/golang/protobuf/ptypes/timestamp"
@@ -542,6 +543,21 @@
 				},
 			},
 		},
+		{
+			in: &metricdata.Point{
+				Time:  startTime.Add(5 * time.Nanosecond),
+				Value: int64(17),
+			},
+			want: &monitoringpb.Point{
+				Interval: &monitoringpb.TimeInterval{
+					StartTime: startTimestamp,
+					EndTime:   timestampProto(startTime.Add(time.Millisecond)),
+				},
+				Value: &monitoringpb.TypedValue{
+					Value: &monitoringpb.TypedValue_Int64Value{Int64Value: 17},
+				},
+			},
+		},
 	}
 
 	for i, tt := range tests {