Align on default service name

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.

## Spec issue
- [x] https://github.com/elastic/apm/issues/572

## Agent issues
- [x] https://github.com/elastic/apm-agent-java/issues/2306
- [x] https://github.com/elastic/apm-agent-dotnet/issues/1585
- [x] https://github.com/elastic/apm-agent-go/issues/1162
- [x] https://github.com/elastic/apm-agent-nodejs/issues/2491
- [x] https://github.com/elastic/apm-agent-python/issues/1425
- [ ] https://github.com/elastic/apm-agent-ruby/issues/1205
- [x] https://github.com/elastic/apm-agent-php/issues/593