X-B3-TraceId support for 8 bytes size as well as 16 bytes size

[rodmeneses]

When trying to use ToHeaders.B3 , which is based on the X-B3-TraceId headers, the code is assuming that the traceId size must be 16 bytes.

The current X-B3-TraceId spec, clearly says that both 8 bytes and 16 bytes size are valid: https://github.com/openzipkin/b3-propagation#traceid

As a workaround, I tried creating my own ToHeaders but I was unable to do so, as the TraceId case class constructor is private. There's an apply Method: apply: Option[TraceId] that will give you None if the size is not 16

def apply(array: Array[Byte]): Option[TraceId] =
    if (array.length == size) Some(new TraceId(array)) else None

where size is initialized at 16 on the TraceId object

I think that

private[trace4cats] class B3ToHeaders extends ToHeaders {

implementation should allow using TraceId of 8 bytes as well of 16 bytes

[janstenpickle]

Thanks @rodmeneses, I think we may still need to make this 16 bytes for some of the downstream exporter implementations. I can check after the 0.14 release.

In the meantime could you try padding the remaining 8bytes? Hopefully that doesn't break the rest of your tracing sytem