Accepts title, description, status, severity, categories, tags,
last_seen_time, and metadata.. A path with an empty value clears the
field, except that title, status, and severity can't be cleared, and
last_seen_time can be cleared only when source_type is MANUAL. categories
and tags are replaced whole. metadata is updated per key: a metadata.
path sets that key from issue.metadata, or removes it when issue.metadata
lacks it, and leaves the other keys alone. A bare metadata path is rejected.
Request for UpdateIssue.