Structure logging to json

[louyuting]

Describe what this PR does / why we need it

format log to json.

2020-09-15T22:14:16.671    /Users/louyuting/Desktop/go/sentinel-golang/logging/logging.go:185    ERROR    test logging function    {"err":"error1","k1":"v1"}

Does this pull request fix one issue?

Describe how you did it

Describe how to verify it

Special notes for reviews

[codecov-commenter]

Codecov Report

Merging #217 into master will decrease coverage by 0.32%.
The diff coverage is 38.91%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
- Coverage   42.63%   42.30%   -0.33%     
==========================================
  Files          74       74              
  Lines        4356     3732     -624     
==========================================
- Hits         1857     1579     -278     
+ Misses       2266     1916     -350     
- Partials      233      237       +4     

Impacted Files	Coverage Δ
api/tracer.go	28.57% <0.00%> (-8.93%)	⬇️
core/base/entry.go	50.00% <0.00%> (+1.28%)	⬆️
core/circuitbreaker/circuit_breaker.go	21.92% <0.00%> (-1.05%)	⬇️
core/flow/slot.go	0.00% <0.00%> (ø)
core/flow/tc_warm_up.go	0.00% <0.00%> (ø)
core/hotspot/cache/lru.go	50.58% <ø> (-3.19%)	⬇️
core/hotspot/concurrency_stat_slot.go	0.00% <0.00%> (ø)
core/hotspot/slot.go	34.14% <0.00%> (+0.81%)	⬆️
core/hotspot/traffic_shaping.go	60.13% <0.00%> (-3.02%)	⬇️
core/log/metric/aggregator.go	25.86% <0.00%> (-3.31%)	⬇️
... and 91 more

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[sczyh30]

LGTM

[sczyh30]

Nice work. Thanks!