Set builder parent span to None when manually setting parent (#26)

## Motivation

When I tried to find a parent span as someone not familiar with the code
I tried using what's in the builder and if there's None I tried the
parent context (which always has trace ID, it just might be invalid).

This change should simplify this for people unfamiliar with the codebase
by getting rid of the trace ID that's not used.

## Solution

Set trace ID in builder to `None`. It has not been used anywhere if
parent_cx contained a valid trace ID so no other changes are needed.

Signed-off-by: David Mládek <[email protected]>

Author: mladedav

src/span_ext.rs

@@ -124,6 +124,7 @@ 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;
                     }
                 });
             }