Add env for http monitor container command (elastic#25746)

* Add ability to set agent.monitoring.http.enabled through container env.

* Fix issue with cfg merge.

* Add to log to help with perms issue.

* Try rotate instead of replace.

* Fix default monitoring port number.

* Fix setPath.

* Add changelog.

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -67,6 +67,7 @@
 - Add error log entry when listener creation fails {issue}23483[23482]
 - Handle case where policy doesn't contain Fleet connection information {pull}25707[25707]
 - Fix fleet-server.yml spec to not overwrite existing keys {pull}25741[25741]
+- Add env for http monitor container command {pull}25746[25746]
 
 ==== New features
 

x-pack/elastic-agent/pkg/agent/cmd/container.go

@@ -24,9 +24,11 @@ import (
 
 	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/agent/configuration"
 
+	"github.com/hectane/go-acl"
 	"github.com/spf13/cobra"
 	"gopkg.in/yaml.v2"
 
+	"github.com/elastic/beats/v7/libbeat/common/file"
 	"github.com/elastic/beats/v7/libbeat/common/transport/tlscommon"
 	"github.com/elastic/beats/v7/libbeat/kibana"
 	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/agent/application/paths"
@@ -270,6 +272,11 @@ func runContainerCmd(streams *cli.IOStreams, cmd *cobra.Command, cfg setupConfig
 		return run(streams, logToStderr)
 	}
 
+	err = prepareAgent(cfg)
+	if err != nil {
+		return err
+	}
+
 	if cfg.Kibana.Fleet.Setup {
 		client, err = kibanaClient(cfg.Kibana)
 		if err != nil {
@@ -325,6 +332,67 @@ func runContainerCmd(streams *cli.IOStreams, cmd *cobra.Command, cfg setupConfig
 	return run(streams, logToStderr)
 }
 
+func prepareAgent(cfg setupConfig) error {
+	// if http monitoring is not enabled then nothing needs to be done
+	if !cfg.Agent.Monitoring.HTTP.Enabled {
+		return nil
+	}
+
+	err := func() error {
+		pathConfigFile := paths.ConfigFile()
+		rawConfig, err := config.LoadFile(pathConfigFile)
+		if err != nil {
+			return fmt.Errorf("failed open config for reading: %s", err)
+		}
+
+		agentCfgOnly := struct {
+			Agent agentConfig `config:"agent"`
+		}{
+			Agent: cfg.Agent,
+		}
+		err = rawConfig.Merge(&agentCfgOnly)
+		if err != nil {
+			return err
+		}
+		data, err := rawConfig.ToMapStr()
+		if err != nil {
+			return err
+		}
+		dataStr, err := yaml.Marshal(data)
+		if err != nil {
+			return err
+		}
+
+		// Matches the same rotation used by enrollment command, but must be duplicated here do to the path
+		// that this code takes.
+		const fsSafeTs = "2006-01-02T15-04-05.9999"
+
+		ts := time.Now()
+		backFilename := pathConfigFile + "." + ts.Format(fsSafeTs) + ".bak"
+		err = file.SafeFileRotate(backFilename, pathConfigFile)
+		if err != nil {
+			return err
+		}
+
+		fd, err := os.OpenFile(pathConfigFile, os.O_CREATE|os.O_WRONLY, 0600)
+		if err != nil {
+			return err
+		}
+		defer fd.Close()
+
+		_, err = fd.Write(dataStr)
+		if err != nil {
+			return err
+		}
+
+		return acl.Chmod(pathConfigFile, 0600)
+	}()
+	if err != nil {
+		return fmt.Errorf("failed to prepare elastic-agent.yml configuration: %s", err)
+	}
+	return nil
+}
+
 func buildEnrollArgs(cfg setupConfig, token string, policyID string) ([]string, error) {
 	args := []string{
 		"enroll", "-f",
@@ -818,6 +886,7 @@ type kibanaAPIKeyDetail struct {
 // setup configuration
 
 type setupConfig struct {
+	Agent       agentConfig       `config:"agent"`
 	Fleet       fleetConfig       `config:"fleet"`
 	FleetServer fleetServerConfig `config:"fleet_server"`
 	Kibana      kibanaConfig      `config:"kibana"`
@@ -831,6 +900,20 @@ type elasticsearchConfig struct {
 	ServiceToken string `config:"service_token"`
 }
 
+type agentConfig struct {
+	Monitoring agentMonitoringConfig `config:"monitoring"`
+}
+
+type agentMonitoringConfig struct {
+	HTTP agentMonitoringHTTPConfig `config:"http"`
+}
+
+type agentMonitoringHTTPConfig struct {
+	Enabled bool   `config:"enabled"`
+	Host    string `config:"host"`
+	Port    string `config:"port"`
+}
+
 type fleetConfig struct {
 	CA              string `config:"ca"`
 	Enroll          bool   `config:"enroll"`
@@ -879,6 +962,15 @@ func defaultAccessConfig() (setupConfig, error) {
 	}
 
 	cfg := setupConfig{
+		Agent: agentConfig{
+			Monitoring: agentMonitoringConfig{
+				HTTP: agentMonitoringHTTPConfig{
+					Enabled: envBool("AGENT_MONITORING_HTTP_ENABLE"),
+					Host:    envWithDefault("0.0.0.0", "AGENT_MONITORING_HTTP_HOST"),
+					Port:    envWithDefault("6791", "AGENT_MONITORING_HTTP_PORT"),
+				},
+			},
+		},
 		Fleet: fleetConfig{
 			CA:              envWithDefault("", "FLEET_CA", "KIBANA_CA", "ELASTICSEARCH_CA"),
 			Enroll:          envBool("FLEET_ENROLL", "FLEET_SERVER_ENABLE"),