Explicitly initialize seelog

The logger package's initialization logic is init() which will be
executed by loading the package.

However some of our programs are using the agent as a library and
haven't configured seelog beforehand. Due to that, loading the package
logs

```
1600397429848419666 [Error] node must have children
```
which breaks the programs.

I think it would be safer to do less on init() and explicitly
initialize the logger from the agent's main().

Signed-off-by: Kazuyoshi Kato <[email protected]>

agent/agent.go

@@ -19,12 +19,14 @@ import (
 	"time"
 
 	"github.com/aws/amazon-ecs-agent/agent/app"
+	"github.com/aws/amazon-ecs-agent/agent/logger"
 )
 
 func init() {
 	rand.Seed(time.Now().UnixNano())
 }
 
 func main() {
+	logger.InitSeelog()
 	os.Exit(app.Run(os.Args[1:]))
 }

agent/logger/log.go

@@ -181,7 +181,9 @@ func init() {
 		MaxFileSizeMB: DEFAULT_MAX_FILE_SIZE,
 		MaxRollCount:  DEFAULT_MAX_ROLL_COUNT,
 	}
+}
 
+func InitSeelog() {
 	if err := seelog.RegisterCustomFormatter("EcsAgentLogfmt", logfmtFormatter); err != nil {
 		seelog.Error(err)
 	}

agent/logger/log_init_test.go

@@ -0,0 +1,23 @@
+// +build unit integration
+
+// Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"). You may
+// not use this file except in compliance with the License. A copy of the
+// License is located at
+//
+//	http://aws.amazon.com/apache2.0/
+//
+// or in the "license" file accompanying this file. This file is distributed
+// on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+// express or implied. See the License for the specific language governing
+// permissions and limitations under the License.
+
+package logger
+
+func init() {
+	// We don't want to call InitSeelog automatically (e.g. calling it from init())
+	// because some internal clients use ECS Agent as a library.
+	// However we want to call InitSeelog automatically from all tests.
+	InitSeelog()
+}