Adding agent.ephemeral_idto metadata

[eyalkoren]

In order to perform aggregations for metrics reported as ever-increasing counters, we need a way to correlate documents related to the same agent. This is required for the agent specific metrics, and maybe also for the breakdown metrics.

We decided to currently go with agent.ephemeral_id, that agents will send as service.agent.ephemeral_id in the metadata.

There is no specific format required for this ID and it should be optional, as currently it will only be implemented by the Java agent.