code: func TraceIDFunc(f IDFunc) TracerOption {
  	return func(o *tracerOptions) *tracerOptions {
  		if f == nil {
  			panic("trace ID function cannot be nil")
  		}
  		o.traceIDFunc = f
  		return o
  	}
  }
docs: // TraceIDFunc is a constructor option that overrides the function used to
  // compute trace IDs.