fluentd_build_info

For many exporters, there's a bit of metadata that is added to the metrics. For example from Prometheus itself
```
# HELP prometheus_build_info A metric with a constant '1' value labeled by version, revision, branch, and goversion from which prometheus was built.
# TYPE prometheus_build_info gauge
prometheus_build_info{branch="HEAD",goversion="go1.9.1",revision="3a7c51ab70fc7615cd318204d3aa7c078b7c5b20",version="1.8.1"} 1
```
or Grafana
```
# HELP grafana_info Information about the Grafana
# TYPE grafana_info gauge
grafana_info{version="4.6.1"} 1
```
Would you accept a pull request to add similar metadata to this plugin?
I was thinking something like

```
fluentd_build_info{rubyversion="??", tdagentversion="??"} 1
```
Would be useful to help see what was deployed