Think there might be an issue with the URL - possibly the order is wrong?
This is the character sequence requiring escapes
OnlineCapacityAnalytics|cpu|alloc|capacityRemaining
The embedded pipe chars '|' correspond to the %7C expressions in the final string - these are how vROPs recognises a metric name
I wonder if the JS engine is putting a '\' character to escape the pipe chars so the pre-URL encoded string is
OnlineCapacityAnalytics\|cpu\|alloc\|capacityRemaining
Then when URL escaping is applied the '\' chars become %5C
Which version of vRO and the REST plugin are you using?