Export otel.status_code tag in Zipkin exporter

[jtescher]

See the spec change for details (may not be exported currently).

[danrusei]

can't the otel.status_code & otel.status_description be added to tags (into_zipkin_span function) ?

tags.insert("otel.status_code".into(), span_data.status_code.to_string());
tags.insert("otel.status_description".into(), span_data.status_message.clone());

Display has to be implemented for StatusCode enum.

[jtescher]

@danrusei yeah should be easy to add, I don't think that anything else needs to format status codes that way, I believe the other exports use the numeric values. Could just add a function from StatusCode -> &'static str that formats the way zipkin expects maybe?

[jtescher]

Resolved in #236