Zipkin - exclude status code if unset

[drexler]

Fixes #355

[codecov]

Codecov Report

Merging #382 (3a3b52d) into master (8353ee2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #382   +/-   ##
=======================================
  Coverage   49.34%   49.34%           
=======================================
  Files          66       66           
  Lines        5326     5326           
=======================================
  Hits         2628     2628           
  Misses       2698     2698           

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[TommyCpp]

Would be nice to indicate the UNSET should never be used here.

opentelemetry-rust/opentelemetry-zipkin/src/exporter/model/mod.rs

Lines 38 to 44 in d157ecf

	fn from_statuscode_to_str(status_code: StatusCode) -> &'static str {
	match status_code {
	StatusCode::Ok => "OK",
	StatusCode::Unset => "UNSET",
	StatusCode::Error => "ERROR",
	}
	}

[drexler]

Would be nice to indicate the UNSET should never be used here.

opentelemetry-rust/opentelemetry-zipkin/src/exporter/model/mod.rs

Lines 38 to 44 in d157ecf

	fn from_statuscode_to_str(status_code: StatusCode) -> &'static str {
	match status_code {
	StatusCode::Ok => "OK",
	StatusCode::Unset => "UNSET",
	StatusCode::Error => "ERROR",
	}
	}

I had intentionally left that with the thinking that the function might be useful in future if there was a need for a stringified version of the StatusCode elsewhere. Anyway, this update should be more flexible. Thanks for the review.

[TommyCpp]

LGTM. Thanks！