Disable exposing metrics server on all interfaces

internal/pkg/agent/application/monitoring/server.go

@@ -20,6 +20,7 @@ import (
 	"github.com/elastic/elastic-agent-libs/config"
 	"github.com/elastic/elastic-agent-libs/monitoring"
 	"github.com/elastic/elastic-agent/internal/pkg/agent/application/coordinator"
+	monitoringCfg "github.com/elastic/elastic-agent/internal/pkg/core/monitoring/config"
 	"github.com/elastic/elastic-agent/pkg/core/logger"
 )
 
@@ -38,6 +39,10 @@ func NewServer(
 		log.Warnf("failed to create monitoring drop: %v", err)
 	}
 
+	if strings.TrimSpace(endpointConfig.Host) == "" {
+		endpointConfig.Host = monitoringCfg.DefaultHost
+	}
+
 	cfg, err := config.NewConfigFrom(endpointConfig)
 	if err != nil {
 		return nil, err

internal/pkg/core/monitoring/config/config.go

@@ -4,10 +4,20 @@
 
 package config
 
-import "time"
+import (
+	"strings"
+	"time"
 
-const defaultPort = 6791
-const defaultNamespace = "default"
+	c "github.com/elastic/elastic-agent-libs/config"
+)
+
+const (
+	defaultPort      = 6791
+	defaultNamespace = "default"
+
+	// DefaultHost is used when host is not defined or empty
+	DefaultHost = "localhost"
+)
 
 // MonitoringConfig describes a configuration of a monitoring
 type MonitoringConfig struct {
@@ -33,6 +43,39 @@ type MonitoringHTTPConfig struct {
 	Buffer  *BufferConfig `yaml:"buffer" config:"buffer"`
 }
 
+// Unpack reads a config object into the settings.
+func (c *MonitoringHTTPConfig) Unpack(cfg *c.C) error {
+	// do not use MonitoringHTTPConfig, it will end up in a loop
+	tmp := struct {
+		Enabled bool          `yaml:"enabled" config:"enabled"`
+		Host    string        `yaml:"host" config:"host"`
+		Port    int           `yaml:"port" config:"port" validate:"min=0,max=65535,nonzero"`
+		Buffer  *BufferConfig `yaml:"buffer" config:"buffer"`
+	}{
+		Enabled: c.Enabled,
+		Host:    c.Host,
+		Port:    c.Port,
+		Buffer:  c.Buffer,
+	}
+
+	if err := cfg.Unpack(&tmp); err != nil {
+		return err
+	}
+
+	if strings.TrimSpace(tmp.Host) == "" {
+		tmp.Host = DefaultHost
+	}
+
+	*c = MonitoringHTTPConfig{
+		Enabled: tmp.Enabled,
+		Host:    tmp.Host,
+		Port:    tmp.Port,
+		Buffer:  tmp.Buffer,
+	}
+
+	return nil
+}
+
 // PprofConfig is a struct for the pprof enablement flag.
 // It is a nil struct by default to allow the agent to use the a value that the user has injected into fleet.yml as the source of truth that is passed to beats
 // TODO get this value from Kibana?
@@ -55,7 +98,7 @@ func DefaultConfig() *MonitoringConfig {
 		MonitorTraces:  false,
 		HTTP: &MonitoringHTTPConfig{
 			Enabled: false,
-			Host:    "localhost",
+			Host:    DefaultHost,
 			Port:    defaultPort,
 		},
 		Namespace:   defaultNamespace,

internal/pkg/core/monitoring/config/config_test.go

@@ -13,6 +13,57 @@ import (
 	"github.com/elastic/elastic-agent/internal/pkg/config"
 )
 
+func TestHost(t *testing.T) {
+	testCases := []struct {
+		name         string
+		config       string
+		expectedHost string
+	}{
+		{"no host", `enabled: true
+logs: true
+metrics: true
+http:
+  enabled: true`, DefaultHost},
+		{"empty host", `enabled: true
+logs: true
+metrics: true
+http:
+  enabled: true
+  host: ""`, DefaultHost},
+		{"whitespace host", `enabled: true
+logs: true
+metrics: true
+http:
+  enabled: true
+  host: "   "`, DefaultHost},
+		{"default", `enabled: true
+logs: true
+metrics: true
+http:
+  enabled: true
+  host: localhost`, DefaultHost},
+		{"custom host", `enabled: true
+logs: true
+metrics: true
+http:
+  enabled: true
+  host: custom`, "custom"},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			c, err := config.NewConfigFrom(tc.config)
+			require.NoError(t, err, "failed to create config")
+
+			cfg := DefaultConfig()
+			err = c.Unpack(&cfg)
+			require.NoError(t, err, "failed to unpack config")
+
+			require.Equal(t, tc.expectedHost, cfg.HTTP.Host)
+		})
+	}
+}
+
 func TestAPMConfig(t *testing.T) {
 	tcs := map[string]struct {
 		in  map[string]interface{}