Add code generator for Go (elastic#249)

This generates Go code based on the Elastic Common Schema.

CHANGELOG.md

@@ -57,6 +57,7 @@ All notable changes to this project will be documented in this file based on the
 * Add fields `source.address`, `destination.address`, `client.address`, and
   `server.address`. #247
 * Add `os.full` to capture full OS name, including version. #259
+* Add generated source code for Go. #249
 
 ### Improvements
 * Improved the definition of the file fields #196

Makefile

@@ -34,6 +34,10 @@ clean:
 	# Clean all markdown files for use-cases
 	find ./use-cases -type f -name '*.md' -not -name 'README.md' -print0 | xargs -0 rm --
 
+# Alias to generate source code for all languages.
+.PHONY: codegen
+codegen: gocodegen
+
 # Build schema.csv from schema files.
 .PHONY: csv
 csv: ve
@@ -66,7 +70,17 @@ fmt: ve
 
 # Alias to generate everything.
 .PHONY: generate
-generate: csv readme template fields
+generate: csv readme template fields codegen
+
+# Generate Go code from the schema.
+.PHONY: gocodegen
+gocodegen:
+	find code/go/ecs -name '*.go' -not -name 'doc.go' | xargs rm
+	cd scripts \
+	  && $(FORCE_GO_MODULES) go run cmd/gocodegen/gocodegen.go \
+	        -version=$(VERSION) \
+	        -schema=../schemas \
+	        -out=../code/go/ecs
 
 # Check Makefile format.
 .PHONY: makelint

code/README.md

@@ -0,0 +1,6 @@
+# Generated Source Code
+
+This directory contains source code that is generated from the Elastic Common
+Schema (ECS).
+
+- [Go](go/ecs/)

code/go/ecs/README.md

@@ -0,0 +1,8 @@
+# Go - Generated Source Code
+
+[![Go Documentation](http://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)][godocs]
+
+[godocs]: http://godoc.org/github.com/elastic/ecs/code/go/ecs
+
+This package contains Go source code that is generated from the Elastic Common
+Schema (ECS).

code/go/ecs/doc.go

@@ -0,0 +1,22 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+// Package ecs contains source code that is generated from the Elastic Common
+// Schema (ECS).
+//
+// https://github.com/elastic/ecs#elastic-common-schema-ecs
+package ecs