add comment about concurrent collection

Author: dashpole

sdk/metric/exemplar/storage.go

@@ -40,6 +40,12 @@ func (r *storage) Collect(dest *[]Exemplar) {
 	*dest = reset(*dest, len(r.measurements), len(r.measurements))
 	var n int
 	for _, val := range r.measurements {
+		// For performance reasons, this iterates over measurements
+		// concurrently with new measurements being written. This means we do
+		// not get a point-in-time snapshot of the state of the reservoir.
+		// This means that for sequential Offer calls, a later Offer call may
+		// be collected and an earlier call not collected if they are written
+		// to different indices.
 		loaded := val.Load()
 		if loaded == nil {
 			continue