Prometheus gauge

Hi!

I'm trying to add a gauge with a constant value in order to see what helm configuration is deployed.
It's an idea that's similar to #40 but just related to my current config release.

After seeing the code https://github.com/fluent/fluent-plugin-prometheus/blob/master/lib/fluent/plugin/prometheus.rb#L197-L207

I think it should be possible to do something like this:
```
# Add constant key in order to be able to use a gauge

@type record_transformer

helm_release {{ .Release.Revision }}

@type prometheus

name fluentd_last_sent
type gauge
key "helm_release"
desc Use for debugging purposes

helm_release_name {{ .Release.Name }}
helm_release_version {{ .Release.Revision }}

```

If I output the record, I can see that the helm_release key is present, but right now if i query the metrics endpoint. I'm not seeing anything:


Am I missing something on the config?