src/Zipkin/Propagation/RequestHeaders.php
public function put(&$carrier, string $key, string $value): void
    {
        $lKey = strtolower($key);
        $carrier = $carrier->withAddedHeader($lKey, $value);
    }
public function put(&$carrier, string $key, string $value): void
    {
        $lKey = strtolower($key);
        $carrier = $carrier->withAddedHeader($lKey, $value);
    }
{@inheritdoc}

@param RequestInterface $carrier
@throws \InvalidArgumentException for invalid header names or values.