Official way to add a Gem inside Logstash for use with Ruby Filter?

At the moment there is no official way to add a Gem on Logstash to make it available for all plugins. As I didn't want to write my own plugin for this (because there is a lot of boilerplate, releases to do... and I only need 10 lines of ruby) here is what I did.

I added this to the root Gemfile (`/usr/share/logstash/Gemfile`):
```
gem "device_detector", "~> 1.0"
```

I ran this to force Logstash updating the gems:
```
/usr/share/logstash/bin/logstash-plugin update logstash-filter-grok
```

Then I can see my Gem in `/usr/share/logstash/vendor/bundle/jruby/1.9/gems/device_detector-1.0.0/` :+1: and use it inside a **ruby** filter for example:

```
filter {
if [user_agent] =~ /.+/ {
ruby {
init => 'require "device_detector"'
code => 'ua = event.get("user_agent")

client = DeviceDetector.new(ua)

if (client.known?)
event.set("[ua_detect][os][name]", client.os_name)
event.set("[ua_detect][os][version]", client.os_full_version)
event.set("[ua_detect][client][name]", client.name)
event.set("[ua_detect][device][type]", client.device_type)
event.set("[ua_detect][device][model]", client.device_name)
end'
}
}
}
```

It works but it feels like a hack...

- is it supposed to work or is there a better way?
- the bundler update is not exposed and I had to use a `bin/logstash-plugin update` call, maybe it should be added as a proper command in the bin directory?

Thanks!
_This way done with Logstash 5.3.1._