Ensure compliance with b3 propagator spec

[jtescher]

See the clarification of the spec here open-telemetry/opentelemetry-specification#1045

[TommyCpp]

I think we almost dodge all the bullets on this one. 😂 The only difference with spec is

MUST attempt to extract B3 encoded using single and multi-header
formats. The single-header variant takes precedence over
the multi-header version.

When extract we will follow users' choice on encoding. But if user select SingleAndMultiHeader, we will try single first. Wonder if we want to change it to always try single format first and fall back to multiple version when single format is not present or invalid.

The rest of the change should already be covered. See details below

MUST preserve a debug trace flag, if received, and propagate
it with subsequent requests. Additionally, an OpenTelemetry implementation
MUST set the sampled trace flag when the debug flag is set.

We set the sampled and debug flag in SpanContext,

opentelemetry-rust/src/sdk/propagation/b3.rs

Line 135 in dbe57cc

"1" => Ok(TRACE_FLAG_DEBUG | TRACE_FLAG_SAMPLED), // debug implies sampled

MUST NOT reuse X-B3-SpanId as the id for the server-side span.

I think we use the result of extraction as remote span context, so we will always generate new span for operation in the application

opentelemetry-rust/src/sdk/propagation/b3.rs

Line 282 in dbe57cc

cx.with_remote_span_context(span_context)

MUST NOT propagate X-B3-ParentSpanId as OpenTelemetry does not support
reusing the same id for both sides of a request.

We don't inject this header

opentelemetry-rust/src/sdk/propagation/b3.rs

Lines 235 to 254 in dbe57cc

	if self.inject_encoding.support(&B3Encoding::MultipleHeader)
	|| self.inject_encoding.support(&B3Encoding::UnSpecified)
	{
	// if inject_encoding is Unspecified, default to use MultipleHeader
	injector.set(
	B3_TRACE_ID_HEADER,
	format!("{:032x}", span_context.trace_id().to_u128()),
	);
	injector.set(
	B3_SPAN_ID_HEADER,
	format!("{:016x}", span_context.span_id().to_u64()),
	);
	if span_context.is_debug() {
	injector.set(B3_DEBUG_FLAG_HEADER, "1".to_string());
	} else if !span_context.is_deferred() {
	let sampled = if span_context.is_sampled() { "1" } else { "0" };
	injector.set(B3_SAMPLED_HEADER, sampled.to_string());
	}
	}