Add host.name in the events

As a solution for elastic#7050, we adding a `host.name` field to
all events. This is duplicate information from `beat.name`,
but is used to avoid the mapping conflict and to slowly
introduce the "host as an object" approach.

To remove the duplication, you can remove `beat.name` like this:

    processors:
      - drop_fields.fields: ["beat.name"]

Closes elastic#7050.

libbeat/publisher/pipeline/module.go

@@ -55,6 +55,9 @@ func Load(
 				"hostname": beatInfo.Hostname,
 				"version":  beatInfo.Version,
 			},
+			Host: common.MapStr{
+				"name": name,
+			},
 		},
 	}
 

libbeat/publisher/pipeline/pipeline.go

@@ -63,6 +63,7 @@ type pipelineProcessors struct {
 	// constructing the clients complete processor
 	// pipeline on connect.
 	beatsMeta common.MapStr
+	hostMeta  common.MapStr
 	fields    common.MapStr
 	tags      []string
 
@@ -93,6 +94,7 @@ type Settings struct {
 type Annotations struct {
 	Beat  common.MapStr
 	Event common.EventMetadata
+	Host  common.MapStr
 }
 
 // WaitCloseMode enumerates the possible behaviors of WaitClose in a pipeline.
@@ -405,6 +407,10 @@ func makePipelineProcessors(
 		p.beatsMeta = common.MapStr{"beat": meta}
 	}
 
+	if meta := annotations.Host; meta != nil {
+		p.hostMeta = common.MapStr{"host": meta}
+	}
+
 	if em := annotations.Event; len(em.Fields) > 0 {
 		fields := common.MapStr{}
 		common.MergeFields(fields, em.Fields.Clone(), em.FieldsUnderRoot)

libbeat/publisher/pipeline/processor.go

@@ -99,10 +99,13 @@ func newProcessorPipeline(
 	// setup 5: client processor list
 	processors.add(localProcessors)
 
-	// setup 6: add beats metadata
+	// setup 6: add beats and metadata
 	if meta := global.beatsMeta; len(meta) > 0 {
 		processors.add(makeAddFieldsProcessor("beatsMeta", meta, needsCopy))
 	}
+	if meta := global.hostMeta; len(meta) > 0 {
+		processors.add(makeAddFieldsProcessor("hostMeta", meta, needsCopy))
+	}
 
 	// setup 7: pipeline processors list
 	processors.add(global.processors)