server: relax test of reported usage
server: relax test of reported usage

returing at-least expected rather than exactly expected allows the test to
succeed even if some other write sneaks in another key between when the
expected numbers are pulled and the tests reporting runs.

Fixes #6318

vaibhavarora/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,petermattis/cockroach,sunya123/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,arjun4084346/cockroach,glycerine/cockroach,paperstreet/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,maxlang/cockroach,vaibhavarora/cockroach,maxlang/cockroach,sunya123/cockroach,cockroachdb/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,glycerine/cockroach,glycerine/cockroach,veteranlu/cockroach,cockroachdb/cockroach,paperstreet/cockroach,maxlang/cockroach,maxlang/cockroach,veteranlu/cockroach,sunya123/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,sunya123/cockroach,paperstreet/cockroach,veteranlu/cockroach,petermattis/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,petermattis/cockroach,vaibhavarora/cockroach,maxlang/cockroach,cockroachdb/cockroach,maxlang/cockroach,maxlang/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,petermattis/cockroach,glycerine/cockroach,sunya123/cockroach,sunya123/cockroach,paperstreet/cockroach,sunya123/cockroach,arjun4084346/cockroach,glycerine/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,petermattis/cockroach,glycerine/cockroach,sunya123/cockroach,veteranlu/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,petermattis/cockroach,glycerine/cockroach,glycerine/cockroach,maxlang/cockroach