proper json response from arcstat (fixes BUGZ-4109)
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This merge request is automatically cherrypicked from !1590 (merged) from source branch https://source.coderefinery.org/dcameron/arc/-/tree/bugz-4109 orginally made by @dcameron .
The original MR went to branch: https://source.coderefinery.org/nordugrid/arc/-/tree/master
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