[8.18] (backport #8261) [bug] Fix monitoring config reloading (#8290)

* [bug] Fix monitoring config reloading (#8261)

Author: mergify[bot]

internal/pkg/agent/application/monitoring/v1_monitor.go

@@ -125,9 +125,15 @@ func (b *BeatsMonitor) Reload(rawConfig *config.Config) error {
 		return nil
 	}
 
-	if err := rawConfig.UnpackTo(&b.config); err != nil {
+	newConfig := monitoringConfig{
+		C: monitoringCfg.DefaultConfig(),
+	}
+
+	if err := rawConfig.UnpackTo(&newConfig); err != nil {
 		return errors.New(err, "failed to unpack monitoring config during reload")
 	}
+
+	b.config = &newConfig
 	return nil
 }
 
@@ -424,6 +430,7 @@ func (b *BeatsMonitor) injectLogsInput(cfg map[string]interface{}, componentInfo
 			"streams":    streams,
 		},
 	}
+
 	inputsNode, found := cfg[inputsKey]
 	if !found {
 		return fmt.Errorf("no inputs in config")

internal/pkg/agent/application/monitoring/v1_monitor_test.go

@@ -20,6 +20,7 @@ import (
 
 	"github.com/elastic/elastic-agent-libs/mapstr"
 	"github.com/elastic/elastic-agent/internal/pkg/agent/application/info"
+	"github.com/elastic/elastic-agent/internal/pkg/config"
 	monitoringcfg "github.com/elastic/elastic-agent/internal/pkg/core/monitoring/config"
 	"github.com/elastic/elastic-agent/pkg/component"
 )
@@ -851,3 +852,29 @@ type AddFields struct {
 	Fields Fields `json:"fields"`
 	Target string `json:"target"`
 }
+
+// This test ensures if any field under [agent.monitoring] is unset,
+// it falls back to the default value defined in monitoringCfg.DefaultConfig()
+func TestMonitorReload(t *testing.T) {
+	// set log and metric monitoring to false
+	monitorcfg := monitoringcfg.DefaultConfig()
+	monitorcfg.MonitorLogs = false
+	monitorcfg.MonitorMetrics = false
+
+	beatsMonitor := New(true, "", monitorcfg, nil)
+	assert.Equal(t, beatsMonitor.config.C.MonitorLogs, false)
+	assert.Equal(t, beatsMonitor.config.C.MonitorLogs, false)
+
+	// unset logs and metrics
+	agentConfig := `
+agent.monitoring:
+  enabled: true
+`
+	conf := config.MustNewConfigFrom(agentConfig)
+	// Reload will set unset fields to default
+	err := beatsMonitor.Reload(conf)
+	require.NoError(t, err)
+
+	assert.Equal(t, beatsMonitor.config.C.MonitorLogs, true)
+	assert.Equal(t, beatsMonitor.config.C.MonitorMetrics, true)
+}