Set auto-gen span's service name to "unknown"

[chrismikehogan]

When a trace is missing a span, we dont know what service is missing, so it is less confusing if we label the service as "unknown" rather than guessing (often incorrectly) that the service's name isthe same as the child span's service name.

[ayansen]

@chrismikehogan This function is used in two cases

1. if there our multiple root spans found in a trace, our best guess is the root span is the one which has the earliest span - which I think holds true
2. There are no root spans found, in that case I'd we can set the root span's service name as unknown

so if you update the code to take in an optional autogenerated root span, service name else use the current behaviour, this pr should be good to merge

[ayansen]

Also you would need to fix the test cases :)