Display the stability status of the Elastic Agent (#17336)

* Display the stability status of the Elastic Agent

The Elastic Agent is currently Alpha quality software and is not ready
to be used in a production environment, this commit make it obvious in
the logs on startup and at enrollment.

Note this doesn't use the cfgwarn helpers for two reasons:

- We aren't using zap in the Elastic Agent code.
- We also need to print on standout.

This also add a "warn" package where the messaging can be edited once.

Fixes: #17328

* Add a changelog entry

x-pack/agent/CHANGELOG.asciidoc

@@ -20,3 +20,4 @@
 - Generate index name in a format type-dataset-namespace {pull}16903[16903]
 - OS agnostic default configuration {pull}17016[17016]
 - Support for config constraints {pull}17112[17112]
+- Display the stability of the agent at enroll and start.  {pull}17336[17336]

x-pack/agent/pkg/agent/application/application.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/agent/application/info"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/agent/errors"
+	"github.com/elastic/beats/v7/x-pack/agent/pkg/agent/warn"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/config"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/core/logger"
 )
@@ -42,6 +43,7 @@ func createApplication(
 	pathConfigFile string,
 	config *config.Config,
 ) (Application, error) {
+	warn.LogNotGA(log)
 
 	log.Info("Detecting execution mode")
 	c := localDefaultConfig()

x-pack/agent/pkg/agent/cmd/enroll.go

@@ -15,6 +15,7 @@ import (
 	c "github.com/elastic/beats/v7/libbeat/common/cli"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/agent/application"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/agent/errors"
+	"github.com/elastic/beats/v7/x-pack/agent/pkg/agent/warn"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/cli"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/config"
 	"github.com/elastic/beats/v7/x-pack/agent/pkg/core/logger"
@@ -44,6 +45,8 @@ func newEnrollCommandWithArgs(flags *globalFlags, _ []string, streams *cli.IOStr
 }
 
 func enroll(streams *cli.IOStreams, cmd *cobra.Command, flags *globalFlags, args []string) error {
+	warn.PrintNotGA(streams.Out)
+
 	config, err := config.LoadYAML(flags.PathConfigFile)
 	if err != nil {
 		return errors.New(err,

x-pack/agent/pkg/agent/warn/warn.go

@@ -0,0 +1,24 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package warn
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/elastic/beats/v7/x-pack/agent/pkg/core/logger"
+)
+
+const message = "The Elastic Agent is currently in Alpha and should not be used in production"
+
+// LogNotGA warns the users in the log that the Elastic Agent is not GA.
+func LogNotGA(log *logger.Logger) {
+	log.Info(message)
+}
+
+// PrintNotGA writes to the received writer that the Agent is not GA.
+func PrintNotGA(output io.Writer) {
+	fmt.Fprintln(output, message)
+}