**Summary**
In preparation for pluggable auto-partitioners, add a new metadata
decorator to replace the four existing ones.
**Additional Context**
"Global" metadata items, those applied to all element on all
partitioners, are applied using a decorator.
Currently there are four decorators where there only needs to be one.
Consolidate those into a single metadata decorator.
One or two additional behaviors of the new decorator will allow us to
remove decorators from delegating partitioners which is a prerequisite
for pluggable auto-partitioners.
