model: align service with ECS

ECS defines [Service](https://www.elastic.co/guide/en/ecs/current/ecs-service.html) fields. APM defines additional service fields, including:
- `service.environment`
- `service.framework`
- `service.language`
- `service.runtime`

We should propose extensions to ECS to accommodate these fields. The names may need changing, e.g. perhaps rename `service.runtime` to just `runtime`, and `service.framework` to `software.framework` or `application.framework`.

We should also consider allowing `service` to be nestable under the [destination](https://www.elastic.co/guide/en/ecs/current/ecs-destination.html) namespace, which would then take place of `span.destination.service.*` fields used for service map metrics.