[errors] Greatly simplify common/errors package.

common/errors/tags.go

Index: common/errors/tags.go
diff --git a/common/errors/tags.go b/common/errors/tags.go
index b80f135c94bda7fa9280987201735df73a0155ed..743cef594427a5b9de26d75afdd4f5787c8edee8 100644
--- a/common/errors/tags.go
+++ b/common/errors/tags.go
@@ -5,11 +5,6 @@
 package errors
 
 type (
-	// TagKeyOrValue is used for functions taking either a TagKey or a TagValue.
-	TagKeyOrValue interface {
-		isTagKeyOrValue()
-	}
-
 	tagDescription struct {
 		description string
 	}
@@ -54,7 +49,7 @@ func TagValueIn(t TagKey, err error) (value interface{}, ok bool) {
 func (t TagValue) GenerateErrorTagValue() TagValue { return t }
 
 // Apply applies this tag value (key+value) directly to the error. This is
-// a shortcut for `errors.Annotate(err).Tag(t).Err()`.
+// a shortcut for `errors.Annotate(err, "").Tag(t).Err()`.
 func (t TagValue) Apply(err error) error {
 	if err == nil {
 		return nil
@@ -63,11 +58,6 @@ func (t TagValue) Apply(err error) error {
 	return a.Tag(t).Err()
 }
 
-// MkTagValue is a shortcut for errors.TagValue{Key: key, Value: val}.
-func MkTagValue(key TagKey, val interface{}) TagValue {
-	return TagValue{key, val}
-}
-
 // BoolTag is an error tag implementation which holds a boolean value.
 //
 // It should be constructed like:
@@ -118,7 +108,7 @@ func (b BoolTag) In(err error) bool {
 //   type SomeType int
 //   type myTag struct { Key errors.TagKey }
 //   func (m myTag) With(value SomeType) errors.TagValue {
-//     return errors.MkTagValue(vm.Key, value)
+//     return errors.TagValue{Key: vm.Key, Value: value}
 //   }
 //   func (m myTag) In(err error) (v SomeType, ok bool) {
 //     d, ok := errors.TagValueIn(m.Key, err)