Parent ID does not Satisfy W3C Propagation

[javiersoto15]

The current W3C propagator in the client is using a 8 rune Hex, making it a 6 byte array. The W3C states the parent-id part of the traceparent has to be an 8 byte array, or a 16 rune hex. This causes for the traceparent to be ignored breaking the tracing when it reaches the service
This is found in the GenerateHeaderBasedOnPropagator() (./client/traceid.go#22)
hex8 := RandHexStringRunes(8)
The PR open makes the change to add a hex16 := RandHexStringRunes(16) variable to further use it in the w3c propagator header