grpcproxy: fix memory leak

[namreg]

use set instead of slice as interval value

fixes #10326

[namreg]

/assign @xiang90

[codecov-io]

Codecov Report

Merging #10327 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #10327     +/-   ##
=========================================
- Coverage   71.75%   71.66%   -0.1%     
=========================================
  Files         391      391             
  Lines       36414    36417      +3     
=========================================
- Hits        26130    26097     -33     
- Misses       8468     8499     +31     
- Partials     1816     1821      +5

Impacted Files	Coverage Δ
proxy/grpcproxy/cache/store.go	95.71% <100%> (+0.19%)	⬆️
auth/options.go	47.5% <0%> (-45%)	⬇️
etcdserver/api/v3rpc/lease.go	67.04% <0%> (-10.23%)	⬇️
etcdserver/util.go	95% <0%> (-3.75%)	⬇️
lease/leasehttp/http.go	62.5% <0%> (-2.95%)	⬇️
etcdserver/api/v3compactor/revision.go	70.58% <0%> (-2.95%)	⬇️
raft/progress.go	94.17% <0%> (-1.95%)	⬇️
mvcc/watchable_store.go	83.5% <0%> (-1.76%)	⬇️
raft/node.go	91.22% <0%> (-1.53%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	72.48% <0%> (-1.35%)	⬇️
... and 18 more

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[namreg]

/assign @gyuho

[xiang90]

lgtm

[jingyih]

cc @jpbetz @wenjiaswe