Check validity of logstash module configuration when `xpack.en… (#12670)

When the `logstash` metricbeat module is enabled and the `xpack.enabled` flag in the module configuration is set to `true`, an exact set of metricsets is required to be configured for Stack Monitoring to work correctly. This PR implements the necessary validation.

metricbeat/module/logstash/logstash.go

@@ -23,10 +23,54 @@ import (
 
 	"github.com/pkg/errors"
 
+	"github.com/elastic/beats/libbeat/common"
 	"github.com/elastic/beats/metricbeat/helper"
 	"github.com/elastic/beats/metricbeat/mb"
 )
 
+func init() {
+	// Register the ModuleFactory function for this module.
+	if err := mb.Registry.AddModule(ModuleName, NewModule); err != nil {
+		panic(err)
+	}
+}
+
+// NewModule creates a new module after performing validation.
+func NewModule(base mb.BaseModule) (mb.Module, error) {
+	if err := validateXPackMetricsets(base); err != nil {
+		return nil, err
+	}
+
+	return &base, nil
+}
+
+// Validate that correct metricsets have been specified if xpack.enabled = true.
+func validateXPackMetricsets(base mb.BaseModule) error {
+	config := struct {
+		Metricsets   []string `config:"metricsets"`
+		XPackEnabled bool     `config:"xpack.enabled"`
+	}{}
+	if err := base.UnpackConfig(&config); err != nil {
+		return err
+	}
+
+	// Nothing to validate if xpack.enabled != true
+	if !config.XPackEnabled {
+		return nil
+	}
+
+	expectedXPackMetricsets := []string{
+		"node",
+		"node_stats",
+	}
+
+	if !common.MakeStringSet(config.Metricsets...).Equals(common.MakeStringSet(expectedXPackMetricsets...)) {
+		return errors.Errorf("The %v module with xpack.enabled: true must have metricsets: %v", ModuleName, expectedXPackMetricsets)
+	}
+
+	return nil
+}
+
 // ModuleName is the name of this module.
 const ModuleName = "logstash"