fix: trace id did not match when propagating invalid context (#55)

## Motivation

Fix following issues
* #54
* #45

## Solution

* Revert https://github.com/tokio-rs/tracing-opentelemetry/pull/26/files
* When an invalid(empty) Context is extracted from the propagator and
passed to OpenTelemetrySpanExt::set_parent(), if none is set for
trace_id, root and child trace id will not match
* I can confirm that this change works in
[reproduction](https://github.com/notheotherben/tracing-opentelemetry-traceid-repro)
that @notheotherben created
* add regression test case which propagate empty context

Author: ymgyt

src/span_ext.rs

@@ -143,7 +143,6 @@ impl OpenTelemetrySpanExt for tracing::Span {
                 get_context.with_context(subscriber, id, move |data, _tracer| {
                     if let Some(cx) = cx.take() {
                         data.parent_cx = cx;
-                        data.builder.trace_id = None;
                     }
                 });
             }

tests/trace_state_propagation.rs

@@ -63,6 +63,24 @@ fn trace_root_with_children() {
     assert_shared_attrs_eq(&spans[0].span_context, &spans[1].span_context);
 }
 
+#[test]
+fn propagate_invalid_context() {
+    let (_tracer, provider, exporter, subscriber) = test_tracer();
+    let propagator = TraceContextPropagator::new();
+    let invalid_cx = propagator.extract(&HashMap::new()); // empty context extracted
+
+    tracing::subscriber::with_default(subscriber, || {
+        let root = tracing::debug_span!("root");
+        root.set_parent(invalid_cx);
+        root.in_scope(|| tracing::debug_span!("child"));
+    });
+
+    drop(provider); // flush all spans
+    let spans = exporter.0.lock().unwrap();
+    assert_eq!(spans.len(), 2);
+    assert_shared_attrs_eq(&spans[0].span_context, &spans[1].span_context);
+}
+
 #[test]
 fn inject_context_into_outgoing_requests() {
     let (_tracer, _provider, _exporter, subscriber) = test_tracer();