[logrus] add output hook to optionally direct trace, info, debug, and…

… warn to stdout

Addresses #26 by providing a new config option, `splitLogging`, to enable certain log severities to go to stdout rather than stderr. I chose to make this optional as I do not want to introduce new behavior into the agent for all users, which may rely on everything going to stderr.

config/config.go

@@ -12,6 +12,7 @@ type Config struct {
 	Watchers       []*WatcherConfig
 	Verbosity      string
 	LegacyLogPaths bool `yaml:"legacyLogPaths"`
+	SplitLogging   bool `yaml:"splitLogging"`
 }
 
 type WatcherConfig struct {

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"fmt"
 	"os"
 	"os/signal"
@@ -22,6 +23,18 @@ import (
 	"k8s.io/client-go/rest"
 )
 
+type OutputSplitter struct{}
+
+// Some systems that process logs expect warn, info, debug, and trace to be on stdout
+// and all error output to go to stderr.
+func (splitter *OutputSplitter) Write(p []byte) (n int, err error) {
+	if bytes.Contains(p, []byte("level=debug")) || bytes.Contains(p, []byte("level=info")) ||
+		bytes.Contains(p, []byte("level=trace")) || bytes.Contains(p, []byte("level=warn")) {
+		return os.Stdout.Write(p)
+	}
+	return os.Stderr.Write(p)
+}
+
 type CmdLineOptions struct {
 	ConfigPath string `long:"config" description:"Path to configuration file" default:"/etc/honeycomb/config.yaml"`
 	Validate   bool   `long:"validate" description:"Validate configuration and exit"`
@@ -66,6 +79,11 @@ func main() {
 		os.Exit(1)
 	}
 
+	if config.SplitLogging {
+		logrus.SetOutput(&OutputSplitter{})
+		logrus.Info("Configured split logging. trace, debug, info, and warn levels will now go to stdout")
+	}
+
 	if config.Verbosity == "debug" {
 		logrus.SetLevel(logrus.DebugLevel)
 	}