Feature request: multiple labels per source

It would be better to be able to route logs from one source to multiple labels, so that it will be much easier to use logs for different purposes, such as storing and monitoring.

Indeed it can be configured using `copy` and `relabel`, but such a combination tends to be indirect and messy, especially when dealing with a lot of sources.
## Example: save all log records and monitor errors in them

With multiple labels in one source, it can be written as below.

``` apache

@type tail
@label @cloudlogging @app_monitor
tag app
path /path/to/app.log
format /(?.+?) +(?.*)/

type google_cloud

@type grep
regexp1 severity ERROR


type sensu
check_status CRITICAL

```

Currently, we must use `copy` and `relabel` instead.

``` apache

@type tail
@label @app_raw
...

@type copy


@type relabel
@label @app_monitor




@type relabel
@label @cloudlogging

...
```