exporter/stackdriver/trace: status code inconsistency between Java and Go

[odeke-em]

Currently the Java implementation sets a trace's status code as 0 for success unconditionally if there is a message or not.

However, the Go implementation doesn't set it and has an explicit check to ensure that we only send the status code if it isn't 0(SUCCESS) or if there is a message

opencensus-go/exporter/stackdriver/trace_proto.go

Lines 66 to 68 in fbaec82

	if s.Status.Code != 0 || s.Status.Message != "" {
	sp.Status = &statuspb.Status{Code: s.Status.Code, Message: s.Status.Message}
	}

We need to be consistent, and for each implementation, what's the rule on what status codes to ignore? If there is one, let's document it too in each implementation otherwise

[odeke-em]

/cc @songy23 @sebright @HailongWen

[sebright]

/cc @bogdandrutu

[rakyll]

What's the difference between having an explicit 0 status vs having none? If it is a significant different, we should also represent this in the APIs by allowing a nil Status case.

If there is no significant difference, then zero value doesn't have to be reported.