Move SDK requirements from semconv to spec

[jpkrohling]

As part of open-telemetry/semantic-conventions#312, we defined a new service.instance.id attribute, which should be added to the spec to track its implementation by SDKs. It should also specify how it can be configured (env vars, file, ...).

In the process, other two attributes were identified as having to be moved as well:

• service.name
• telemetry.sdk group

This is related to this whole section: https://github.com/open-telemetry/semantic-conventions/tree/main/docs/resource#semantic-attributes-with-sdk-provided-default-value

[jpkrohling]

To be clear: what needs to be "moved" is the statement that those attributes MUST be populated by the SDK.

[jpkrohling]

Further comment about the service.instance.id: the spec needs to account for applications running in front of application servers like unicorn. A special value for the service.instance.id could be set by users (uuidv1?), telling the SDK that a stable UUID isn't desirable.