fix: trace id did not match when propagating invalid context

[ymgyt]

Motivation

Fix following issues

• Regression with 0.20 regarding remote contexts #54
• Correct way to handle trace context propagation when a context might not be present? #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 that @notheotherben created
• add regression test case which propagate empty context

[jtescher]

Nice

[djc]

@jtescher can we get this published to crates.io? We're also running into this problem at work.

(Let me know if/how I can help.)

[jtescher]

Will be available once #56 is approved.