Add support for alias field type to fields.yml

In elastic/elasticsearch#23714 Elasticsearch implemented the alias field type. This can be used in fields.yml as following:

```
- name: a.b
  type: alias
  path: a.c
```

`a.b` will be the alias for `a.c`.

libbeat/common/field.go

@@ -52,6 +52,7 @@ type Field struct {
 	DocValues             *bool       `config:"doc_values"`
 	CopyTo                string      `config:"copy_to"`
 	IgnoreAbove           int         `config:"ignore_above"`
+	AliasPath             string      `config:"path"`
 
 	// Kibana specific
 	Analyzed     *bool  `config:"analyzed"`

libbeat/template/processor.go

@@ -62,6 +62,8 @@ func (p *Processor) Process(fields common.Fields, path string, output common.Map
 			mapping = p.object(&field)
 		case "array":
 			mapping = p.array(&field)
+		case "alias":
+			mapping = p.alias(&field)
 		case "group":
 			var newPath string
 			if path == "" {
@@ -242,6 +244,13 @@ func (p *Processor) array(f *common.Field) common.MapStr {
 	return properties
 }
 
+func (p *Processor) alias(f *common.Field) common.MapStr {
+	properties := getDefaultProperties(f)
+	properties["type"] = "alias"
+	properties["path"] = f.AliasPath
+	return properties
+}
+
 func (p *Processor) object(f *common.Field) common.MapStr {
 	dynProperties := getDefaultProperties(f)
 

libbeat/template/processor_test.go

@@ -86,6 +86,10 @@ func TestProcessor(t *testing.T) {
 			output:   p.array(&common.Field{Type: "array", Index: &falseVar, ObjectType: "keyword"}),
 			expected: common.MapStr{"index": false, "type": "keyword"},
 		},
+		{
+			output:   p.alias(&common.Field{Type: "alias", AliasPath: "a.b"}),
+			expected: common.MapStr{"path": "a.b", "type": "alias"},
+		},
 		{
 			output: p.object(&common.Field{Type: "object", Enabled: &falseVar}),
 			expected: common.MapStr{

libbeat/template/testdata/fields.yml

@@ -20,3 +20,7 @@
     - name: array_disabled
       type: array
       enabled: false
+
+    - name: alias
+      type: alias
+      path: keyword