Use attribute Sets instead of KeyValues for export data (#3012)

Attribute Sets have stronger guarantees about the uniqueness of their
keys and more functionality. We already ensure attributes are stored as
Sets by the aggregator which will produce these data types. Instead of
converting to a KeyValue slice, keep the data as a Set.

Any user of the data can always call the ToSlice method to use the data
as a slice of KeyValues.

sdk/metric/export/data.go

@@ -87,8 +87,9 @@ func (Sum) privateAggregation() {}
 
 // DataPoint is a single data point in a timeseries.
 type DataPoint struct {
-	// Attributes is the set of key value pairs that uniquely identify the timeseries.
-	Attributes []attribute.KeyValue
+	// Attributes is the set of key value pairs that uniquely identify the
+	// timeseries.
+	Attributes attribute.Set
 	// StartTime is when the timeseries was started. (optional)
 	StartTime time.Time
 	// Time is the time when the timeseries was recorded. (optional)
@@ -126,8 +127,9 @@ func (Histogram) privateAggregation() {}
 
 // HistogramDataPoint is a single histogram data point in a timeseries.
 type HistogramDataPoint struct {
-	// Attributes is the set of key value pairs that uniquely identify the timeseries.
-	Attributes []attribute.KeyValue
+	// Attributes is the set of key value pairs that uniquely identify the
+	// timeseries.
+	Attributes attribute.Set
 	// StartTime is when the timeseries was started.
 	StartTime time.Time
 	// Time is the time when the timeseries was recorded.