I have add a choice 'service' for tag_for parameters.

I have add a choice 'service' for tag_for parameters.
Would you please merge my commit to your branch?

Now, we can use tag for service name.
It is a sample that the service name to be 'MyWeb'.

### input tag
metrics.site.MyWeb

### Configuration
`````
<match metrics.**>
  type growthforecast
  gfapi_url http://localhost:5125/api/
  section   response_time
  remove_prefix metrics.site
  name_keys avg
</match>
`````

### the result at GrowthForecast
service: MyWeb
section: response_time