Each tag consists of a tag key and a tag value. Tag keys and tag values
are both required, but tag values can be empty (null) strings.
You can only use a tag key once for each CMK. If you use the tag key
again, AWS KMS replaces the current tag value with the specified value.
For information about the rules that apply to tag keys and tag values,
see User-Defined Tag Restrictions
in the AWS Billing and Cost Management User Guide.
The result of this operation varies with the key state of the CMK. For
details, see How Key State Affects Use of a Customer Master Key
in the AWS Key Management Service Developer Guide.