model: align service with ECS

[axw]

ECS defines Service 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 namespace, which would then take place of span.destination.service.* fields used for service map metrics.

[graphaelli]

relates to elastic/ecs#891