Add Log Interface

[arugal]

Background

Now GRPCReporter can only use log.Logger to output logs, but there are many good log libraries, such as zap, logrus, I think we should design a logging interface and let the user decide which one to use.

Design

type Log interface {
	// Info logs to the INFO log.
	Info(args ...interface{})
	// Infof logs to the INFO log.
	Infof(format string, args ...interface{})
	// Warning logs to the WARNING and INFO logs.
	Warn(args ...interface{})
	// Warningf logs to the WARNING and INFO logs.
	Warnf(format string, args ...interface{})
	// Error logs to the ERROR, WARNING, and INFO logs.
	Error(args ...interface{})
	// Errorf logs to the ERROR, WARNING, and INFO logs.
	Errorf(format string, args ...interface{})
}

Use Case

• golang/log (default)

	logger := log.New(os.Stderr, "go2sky", log.LstdFlags)

	reporter, _ := NewGRPCReporter("127.0.0.1:11800", WithLogger(logger))

• zap

	logger, _ := zap.NewProduction()
	defer logger.Sync() // flushes buffer, if any
	sugar := logger.Sugar()

	reporter, _ := NewGRPCReporter("127.0.0.1:11800", WithLog(sugar))

• logrus

	logger := logrus.New()
	
	reporter, _ := NewGRPCReporter("127.0.0.1:11800", WithLog(logger))

[codecov-commenter]

Codecov Report

Merging #147 (3848d35) into master (a12c5be) will increase coverage by 0.09%.
The diff coverage is 35.71%.

❗ Current head 3848d35 differs from pull request most recent head 40f8f27. Consider uploading reports for the commit 40f8f27 to get more accurate results

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   70.64%   70.73%   +0.09%     
==========================================
  Files          18       18              
  Lines         930      933       +3     
==========================================
+ Hits          657      660       +3     
  Misses        225      225              
  Partials       48       48              

Impacted Files	Coverage Δ
reporter/grpc.go	56.14% <35.71%> (+0.58%)	⬆️

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[wu-sheng]

Could you introduce what is this? I can't see the update in readme doc.

[arugal]

Could you introduce what is this? I can't see the update in readme doc.

Description has been added for this PR