FR: add metadata to generated samples

[sofisl]

// sample-metadata:
//   title: {{ api.mainServiceName }} {{ service.name.toPascalCase() }} {{ method.name.toCamelCase() }}
//   description: {{- util.printCommentsForMethodOneLine(method) }}
//   generated: true
//   api: {{ api.mainServiceName.toSnakeCase() }}
//   api_version: {{ api.naming.version }}
//   rpcName: {{ service.name.toCamelCase() }}.{{ method.name.toCamelCase() }}
//   serviceName: {{ api.mainServiceName }}

[sofisl]

Closing this for now, will follow Amanda's PRD