Fluent interface for Span

[memboc]

Proposal

Would be great to make fluent interface for Span. It's very popular practice

now:

        $scope->getSpan()->setTag(Tags::ERROR, true);
        $scope->getSpan()->setTag(Tags::STATUS_CODE, $httpCode);
        $scope->getSpan()->log(['message' => $response]);

fluent interface:

        $scope->getSpan()
            ->setTag(Tags::ERROR, true)
            ->setTag(Tags::STATUS_CODE, $httpCode)
            ->log(['message' => $response]);