Added NoDebugLogger that doesn't format debug logs

fmt.Spritnf() is too expensive to call and debug logs are usually
disabled in prod, so we don't want to waste time formatting them.

Author: colega

no_debug_logger.go

@@ -0,0 +1,12 @@
+package log
+
+// NoDebugLogger embeds a Logger, but in calls to debug functions it does nothing.
+// It avoids doing fmt.Sprintf() for those calls as they will be discarded anyways.
+// This makes those calls like 50 times faster (see benchmark file)
+type NoDebugLogger struct {
+	Logger
+}
+
+func (NoDebugLogger) Debug(args ...interface{})                 {}
+func (NoDebugLogger) Debugf(format string, args ...interface{}) {}
+func (NoDebugLogger) Debugln(args ...interface{})               {}

no_debug_logger_bench_test.go

@@ -0,0 +1,50 @@
+package log
+
+import (
+	"testing"
+)
+
+/*
+$ go test -bench=. ./...
+     | goos: darwin
+     | goarch: amd64
+     | pkg: github.com/cabify/go-logging
+     | BenchmarkPlainLoggerDebugSpeed-4      	30000000	        45.9 ns/op
+     | BenchmarkNoDebugLoggerDebugSpeed-4    	2000000000	         0.33 ns/op
+     | BenchmarkPlainLoggerDebugfSpeed-4     	20000000	        80.8 ns/op
+     | BenchmarkNoDebugLoggerDebugfSpeed-4   	50000000	        36.5 ns/op
+*/
+
+type someStruct struct {
+	value float64
+}
+
+var (
+	structValue           = someStruct{value: 10.0}
+	plainLoggerInstance   = NewLogger("test")
+	noDebugLoggerInstance = NoDebugLogger{Logger: plainLoggerInstance}
+)
+
+func BenchmarkPlainLoggerDebugSpeed(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		plainLoggerInstance.Debug("Something")
+	}
+}
+
+func BenchmarkNoDebugLoggerDebugSpeed(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		noDebugLoggerInstance.Debug("Something")
+	}
+}
+
+func BenchmarkPlainLoggerDebugfSpeed(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		plainLoggerInstance.Debugf("Something %v, %d", structValue, i)
+	}
+}
+
+func BenchmarkNoDebugLoggerDebugfSpeed(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		noDebugLoggerInstance.Debugf("Something %v, %d", structValue, i)
+	}
+}