Add ephemeral_id to Logstash and Pipeline

[ph]

To offer better metrics will need to better track the lifecycle of pipelines and Logstash instances when they are restarted or created. To do so we will add ephemeral_id at the Logstash level and at the Pipeline level and theses information will be available when we gather the metrics.

logstash.ephemeral_id
pipeline.ephemeral_id

We will need to make them available for x-pack and in the API.

[ph]

cc @andrewvc

[ph]

Concerning the implementation do we want to use SecureRandom.uuid like we do with the persisted UUID or SecureRandom.urlsafe_base64?

https://ruby-doc.org/stdlib-1.9.2/libdoc/securerandom/rdoc/SecureRandom.html

[ph]

For consistency, I will probably go with SecureRandom.uuid