census: add span kind and simplify span names

[gfelbing]

This PR sets the span kind on client/server spans and simplifies generation of span names.

@voidzcy reverted the initial approach of simply adding the kind due to internal opencensus implementation relying on the convention Span without kind needs to be in the format of "[Recv|Sent].Namespace.Service.Method" and Span with kind needs to be in the format of "Namespace.Service/Method"

As MethodDescriptor#getFullMethodName already returns the correct naming convention for spans with kind, the method generateTraceSpanName can be dropped when adding the span kind.

cc @creamsoup

[voidzcy]

Thanks for polishing this.

[voidzcy]

We would not merge it now as we are unsure if this will break any internal monitoring tools. We will get back to you when we figured it out.

[creamsoup]

this changes seems more desirable way. However, since the metric name is API, we can't change it unless we provide an migration plan. e.g. support both for a while to migrate.
I don't know how users use the metrics, but some monitoring / data collection can be broken after this change.
cc @zhangkun83 who has knowledge of census in general.

[ejona86]

At this point, OpenTelemetry design is making progress and we are able to change names there. We'll just leave OpenCensus the way it is; little benefit to touching it and making breaking something at thi spoint.