libbeat/beat: introduce Beat.OutputConfigReloader (#28048)

Introduce Beat.OutputConfigReloader, an optional
reload.Reloadable that may be set by a beat.Creator
in order to watch output config changes.

CHANGELOG-developer.next.asciidoc

@@ -120,3 +120,4 @@ The list below covers the major changes between 7.0.0-rc2 and master only.
 - Add sorting to array fields for generated data files (*-generated.json) {pull}25320[25320]
 - Update to go-concert 0.2.0 {pull}27162[27162]
 - Update Go version to 1.16.5. {issue}26182[26182] {pull}26186[26186]
+- Introduce `libbeat/beat.Beat.OutputConfigReloader` {pull}28048[28048]

libbeat/beat/beat.go

@@ -19,6 +19,7 @@ package beat
 
 import (
 	"github.com/elastic/beats/v7/libbeat/common"
+	"github.com/elastic/beats/v7/libbeat/common/reload"
 	"github.com/elastic/beats/v7/libbeat/instrumentation"
 	"github.com/elastic/beats/v7/libbeat/keystore"
 	"github.com/elastic/beats/v7/libbeat/management"
@@ -63,6 +64,12 @@ type Beat struct {
 	//      pipeline and ML jobs.
 	Config *BeatConfig // Common Beat configuration data.
 
+	// OutputConfigReloader may be set by a Creator to watch for output config changes.
+	//
+	// This reloader is called in addition to libbeat's internal output reloader, which
+	// is responsible for reconfiguring Publisher.
+	OutputConfigReloader reload.Reloadable
+
 	BeatConfig *common.Config // The beat's own configuration section
 
 	Fields []byte // Data from fields.yml

libbeat/cmd/instance/beat.go

@@ -861,6 +861,11 @@ func (b *Beat) indexSetupCallback() elasticsearch.ConnectCallback {
 
 func (b *Beat) makeOutputReloader(outReloader pipeline.OutputReloader) reload.Reloadable {
 	return reload.ReloadableFunc(func(config *reload.ConfigWithMeta) error {
+		if b.OutputConfigReloader != nil {
+			if err := b.OutputConfigReloader.Reload(config); err != nil {
+				return err
+			}
+		}
 		return outReloader.Reload(config, b.createOutput)
 	})
 }