Move addFields to ObjectEncoder to let logger#With be extendible

[lysu]

1. What did you do?

we write a TextEncoder to let log be formated as

[2019/03/04 23:00:05.943 +08:00] [ERROR] [log_test.go:209] [common=comval] [k=v]

but found that With method cannot add [ ]

newLogger := logger.With(zap.String("c", "c1"))
newLogger.Error("", zap.String("k", "v"))

will got

[2019/03/04 23:00:05.943 +08:00] [ERROR] [log_test.go:209],c=c1 [k=v]

2. What's the problem

found that addFields at

https://github.com/uber-go/zap/blob/master/zapcore/field.go#L197

maybe better move to objectEncoder at

https://github.com/uber-go/zap/blob/master/zapcore/encoder.go#L247

so transfer the responsibility of how to print multiple field to objectEncoder, so other people can customize it, eg. we can add [ ] sourrounding each field even them add via logger#With

3. What is changed

move addFields to ObjectEncoder and let current implements it.

ref pingcap/log#3, pingcap/tidb#9548

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #685 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #685      +/-   ##
==========================================
+ Coverage   97.38%   97.38%   +<.01%     
==========================================
  Files          40       40              
  Lines        2102     2105       +3     
==========================================
+ Hits         2047     2050       +3     
  Misses         47       47              
  Partials        8        8

Impacted Files	Coverage Δ
zapcore/field.go	100% <ø> (ø)	⬆️
zapcore/encoder.go	83.13% <ø> (ø)	⬆️
zapcore/console_encoder.go	96.49% <100%> (ø)	⬆️
zapcore/core.go	94.59% <100%> (ø)	⬆️
zapcore/memory_encoder.go	98.71% <100%> (+0.05%)	⬆️
zapcore/json_encoder.go	100% <100%> (ø)	⬆️

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[lysu]

@akshayjshah PTAL thx~

[prashantv]

Thanks for the contribution @lysu.

Unfortunately this change is not backwards compatible, since you're modifying the interface, so we won't be able to accept this change in zap 1.x

[lysu]

@prashantv we will take another temp way to solve this problem in current and wait for the 2.x, thx :D