api: move SpanKind display format impl to jaeger (#758)

This format implementation is jaeger specific and not defined in the
otel spec.

opentelemetry-api/src/trace/span.rs

@@ -1,7 +1,6 @@
 use crate::{trace::SpanContext, KeyValue};
 use std::borrow::Cow;
 use std::error::Error;
-use std::fmt;
 use std::time::SystemTime;
 
 /// The interface for a single operation within a trace.
@@ -226,18 +225,6 @@ pub enum SpanKind {
     Internal,
 }
 
-impl fmt::Display for SpanKind {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            SpanKind::Client => write!(f, "client"),
-            SpanKind::Server => write!(f, "server"),
-            SpanKind::Producer => write!(f, "producer"),
-            SpanKind::Consumer => write!(f, "consumer"),
-            SpanKind::Internal => write!(f, "internal"),
-        }
-    }
-}
-
 /// The code representation of the status of a [`Span`].
 ///
 /// These values form a total order: Ok > Error > Unset. This means that setting

opentelemetry-jaeger/src/exporter/mod.rs

@@ -764,7 +764,7 @@ fn build_span_tags(
     }
 
     if !user_overrides.span_kind && kind != SpanKind::Internal {
-        tags.push(Key::new(SPAN_KIND).string(kind.to_string()).into());
+        tags.push(Key::new(SPAN_KIND).string(format_span_kind(kind)).into());
     }
 
     if status_code != StatusCode::Unset {
@@ -792,6 +792,16 @@ fn build_span_tags(
     tags
 }
 
+fn format_span_kind(kind: SpanKind) -> &'static str {
+    match kind {
+        SpanKind::Client => "client",
+        SpanKind::Server => "server",
+        SpanKind::Producer => "producer",
+        SpanKind::Consumer => "consumer",
+        SpanKind::Internal => "internal",
+    }
+}
+
 const ERROR: &str = "error";
 const SPAN_KIND: &str = "span.kind";
 const OTEL_STATUS_CODE: &str = "otel.status_code";