span_context.rb

require 'base64'

class SpanContext
  def initialize(trace_id: '', span_id: '')
    raise RuntimeError 'trace_id must be 16 bytes' unless trace_id.size == 16
    raise RuntimeError 'span_id must be 8 bytes' unless span_id.size == 8

    @version  = [0]
    @trace_id = [0] + trace_id.bytes # field id 0 + trace_id
    @span_id  = [1] + span_id.bytes # field id 1 + span_id
    @options  = [2, 1] # field id 2 + trace options
  end

  def to_bytes
    @version + @trace_id + @span_id + @options
  end

  def pack
    to_bytes.pack('C*')
  end
end

span_context = SpanContext.new(trace_id: '@ABCDEFGHIJKLMNO', span_id: 'abcdefgh')
print Base64.urlsafe_encode64(span_context.pack, padding: false)
print "\n"