[META 571] Spec: Align on default service name

[felixbarny]

Write Spec for:
As defined in #499, agents don't require users to configure the service_name. Ideally, the a descriptive value can be inferred form the application. But in some cases that's not possible. In these cases, agents must now align on the naming scheme for the default service name. This enables the UI to show inline help for services that it identifies as unnamed services based on the naming scheme.

[SylvainJuge]

@felixbarny when aligning the agents on this new default for service.name, should we consider this as a:

• "breaking change"
• "potentially breaking change" ?
• "not a breaking change"

My first thought would be to not consider this as a breaking change:

• if a user does not set service.name, they get a default (and likely meaningless) value
• as a result using a default value is quite unlikely to be used for filtering or in visualizations.

[felixbarny]

I agree to consider this a non-breaking change.