Minor: Small fixes for add_kubernetes_metadata (#7466)

* When we fail to build an indexer or matcher we should not add them to
the execution.

* Empty() predicates should be behind a mutex

CHANGELOG.asciidoc

@@ -73,6 +73,7 @@ https://github.com/elastic/beats/compare/v6.2.3...master[Check the HEAD diff]
 - Allow index-pattern only setup when setup.dashboards.only_index=true. {pull}7285[7285]
 - Fix duplicating dynamic_fields in template when overwriting the template. {pull}7352[7352]
 - Fix a panic on the Dissect processor when we have data remaining after the last delimiter. {pull}7449[7449]
+- When we fail to build a Kubernetes' indexer or matcher we produce a warning but we don't add them to the execution. {pull}7466[7466]
 
 *Auditbeat*
 

libbeat/processors/add_kubernetes_metadata/indexers.go

@@ -60,7 +60,7 @@ type Indexers struct {
 // IndexerConstructor builds a new indexer from its settings
 type IndexerConstructor func(config common.Config, metaGen kubernetes.MetaGenerator) (Indexer, error)
 
-// NewIndexers  builds indexers object
+// NewIndexers builds indexers object
 func NewIndexers(configs PluginConfig, metaGen kubernetes.MetaGenerator) *Indexers {
 	indexers := []Indexer{}
 	for _, pluginConfigs := range configs {
@@ -74,6 +74,7 @@ func NewIndexers(configs PluginConfig, metaGen kubernetes.MetaGenerator) *Indexe
 			indexer, err := indexFunc(pluginConfig, metaGen)
 			if err != nil {
 				logp.Warn("Unable to initialize indexing plugin %s due to error %v", name, err)
+				continue
 			}
 
 			indexers = append(indexers, indexer)
@@ -113,6 +114,8 @@ func (i *Indexers) GetMetadata(pod *kubernetes.Pod) []MetadataIndex {
 
 // Empty returns true if indexers list is empty
 func (i *Indexers) Empty() bool {
+	i.RLock()
+	defer i.RUnlock()
 	if len(i.indexers) == 0 {
 		return true
 	}

libbeat/processors/add_kubernetes_metadata/matchers.go

@@ -62,6 +62,7 @@ func NewMatchers(configs PluginConfig) *Matchers {
 			matcher, err := matchFunc(pluginConfig)
 			if err != nil {
 				logp.Warn("Unable to initialize matcher plugin %s due to error %v", name, err)
+				continue
 			}
 
 			matchers = append(matchers, matcher)
@@ -89,6 +90,8 @@ func (m *Matchers) MetadataIndex(event common.MapStr) string {
 }
 
 func (m *Matchers) Empty() bool {
+	m.RLock()
+	defer m.RUnlock()
 	if len(m.matchers) == 0 {
 		return true
 	}