[errors] de-specialize Transient in favor of Tags.

common/retry/transient.go

Index: common/retry/transient.go
diff --git a/common/retry/transient.go b/common/retry/transient.go
index fa641d446df9574cf752afbed012b4e1a08eb44a..6fbe3b37d6153bbd53fd1fffa58e2c8dc2f2c1c4 100644
--- a/common/retry/transient.go
+++ b/common/retry/transient.go
@@ -12,15 +12,13 @@ import (
 )
 
 // transientOnlyIterator is an Iterator implementation that only retries errors
-// if they are transient.
-//
-// (See errors.IsTransient).
+// if they are tagged with `retry.Tag`.
 type transientOnlyIterator struct {
 	Iterator // The wrapped Iterator.
 }
 
 func (i *transientOnlyIterator) Next(ctx context.Context, err error) time.Duration {
-	if !errors.IsTransient(err) {
+	if !Tag.In(err) {
 		return Stop
 	}
 	return i.Iterator.Next(ctx, err)
@@ -38,3 +36,11 @@ func TransientOnly(f Factory) Factory {
 		return &transientOnlyIterator{it}
 	})
 }
+
+// Tag is used to indicate that an error should be retried by this package. Use
+// like:
+//   err = retry.Tag.Apply(err)
+//
+// This allows libraries to preserve the original error, but indicate to the
+// retry library that the error may be transient.
+var Tag = errors.NewBoolTag("this error may go away with a retry")