Document that ExportView is not called concurrently (#864)

This allows Exporter implementations to avoid unnecessary internal
synchronization.

Author: Ramon Nogueira

stats/view/export.go

@@ -27,6 +27,9 @@ var (
 // Exporter takes a significant amount of time to
 // process a Data, that work should be done on another goroutine.
 //
+// It is safe to assume that ExportView will not be called concurrently from
+// multiple goroutines.
+//
 // The Data should not be modified.
 type Exporter interface {
 	ExportView(viewData *Data)