[prometheus-to-sd] Gauges are always interpreted as int64

For example, if I import the following

```
# HELP jenkins_health_check_count Generated from Dropwizard metric import (metric=jenkins.health-check.count, type=jenkins.metrics.api.Metrics$HealthChecker$1)
# TYPE jenkins_health_check_count gauge
jenkins_health_check_count 4.0
```

And then the imported metric looks like the following in stackdriver

I believe this can be fixed with a slight tweak to `extractValueType()`. But maybe there is a smarter way to fix this.