feat:add logger support

[jtyjty99999]

https://github.com/eggjs/egg/issues/648

[mention-bot]

@jtyjty99999, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fengmk2, @csbun and @fangk to be potential reviewers.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@e8d8056). Click here to learn what that means. The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #18   +/-   ##
=========================================
  Coverage          ?   98.94%           
=========================================
  Files             ?        6           
  Lines             ?      285           
  Branches          ?       41           
=========================================
  Hits              ?      282           
  Misses            ?        3           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/client.js	95.38% <100%> (ø)
lib/operator.js	100% <100%> (ø)

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[fengmk2]

参照 https://github.com/eggjs/egg/issues/235 模式实现吧。

[jtyjty99999]

@fengmk2 只暴露request和response事件？

[fengmk2]

logger 基于这两个事件来打印，如果配置了开启 logger，就打印sql 执行时间

[atian25]

ping @jtyjty99999 这个怎么样了

[fengmk2]

https://github.com/ali-sdk/ali-rds/pull/104