Fixed an issue where variables were printed with the wrong placeholder

[haoyu234]

Here are some sample outputs:

before

DEBU[0000] adding server &{f8da4226-e555-d75f-bf35-e7e3f9e84945 chat map[] %!s(bool=true) h.dev}  source=pitaya
DEBU[0000] subs channel size: %!d(float64=0), max: %!d(float64=0), dropped: 0  source=tank

after

DEBU[0000] adding server &{5589a691-ad56-4d31-bca2-55d742d97ece connector map[] true h.dev}  source=pitaya
DEBU[0000] subs channel size: 0, max: 0, dropped: 0      source=pitaya

[coveralls]

Pull Request Test Coverage Report for Build 5830029887

• 1 of 2 (50.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 62.238%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cluster/etcd_service_discovery.go	0	1	0.0%

Totals
Change from base Build 5658434483:	0.0%
Covered Lines:	4783
Relevant Lines:	7685

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💛 - Coveralls

[felipejfc]

thanks for the contribution