[log] Fix #110 simplify log registry

[at15]

TODO

• [x] #110 simplify registry
• [ ] #23 control log level using http API
• [x] Fix #60 allow set log level using env var
• [ ] allow set log level using flags
• [x] change Add, might just return copy instead of doing inplace update
• [ ] try skip escape analysis to avoid overhead of using io.Writer in logging library? https://github.com/golang/go/blob/go1.12.1/src/strings/builder.go#L20-L30

Done

• [x] use go mod for benchmark

[codecov[bot]]

Codecov Report

Merging #111 into master will decrease coverage by 1.34%. The diff coverage is 30.64%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   52.35%   51.01%   -1.35%     
==========================================
  Files          59       56       -3     
  Lines        2183     2074     -109     
==========================================
- Hits         1143     1058      -85     
+ Misses        952      933      -19     
+ Partials       88       83       -5

Impacted Files	Coverage Δ
log/handler.go	66% <ø> (ø)	:arrow_up:
generator/pkg.go	0% <ø> (ø)	:arrow_up:
log/pkg.go	0% <0%> (ø)
log/testing.go	90.47% <100%> (ø)	:arrow_up:
log/field.go	73.68% <100%> (-1.32%)	:arrow_down:
log/logger.go	40.36% <100%> (+9.67%)	:arrow_up:
log/logger_identity.go	68.42% <100%> (ø)	:arrow_up:
log/level.go	42.42% <15.38%> (-17.58%)	:arrow_down:
log/registry.go	17.24% <7.84%> (+2.65%)	:arrow_up:
log/logger_factory.go	93.1% <91.66%> (+2.4%)	:arrow_up:
... and 3 more

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