Propagate correct span ID via W3C context (#2408)

The W3C context propagation uses the wrong span ID right now. That
causes all spans emitted by linkerd-proxy to be siblings rather than
children of their original parent.

This only applies to W3C as far as I can tell, because the B3
propagation uses the span ID correctly.

Signed-off-by: Willi Schönborn <[email protected]>

Author: whiskeysierra

linkerd/trace-context/src/propagation/w3c.rs

@@ -25,7 +25,7 @@ pub fn increment_http_span_id<B>(request: &mut http::Request<B>, context: &Trace
         buf.push('-');
         buf.push_str(&hex::encode(context.trace_id.as_ref()));
         buf.push('-');
-        buf.push_str(&hex::encode(context.parent_id.as_ref()));
+        buf.push_str(&hex::encode(span_id.as_ref()));
         buf.push('-');
         buf.push_str(&hex::encode(vec![context.flags.0]));
         buf