Support Symfony Messenger

[imjoehaines]

Goal

Building off of #89, this PR adds support for Symfony Messenger. Primarily this involves listening for a WorkerMessageFailedEvent (caused by unhandled errors in a job) and notifying when this happens. However, this doesn't quite work because of batch sending mode , where we flush the queue of events on shutdown. Worker processes are generally long lived and therefore won't trigger this flush often, which means events would end up not being delivered

To fix this, we flush the queue after a WorkerMessageFailedEvent, which will send the event from the failure and any manual notify calls. We also flush the queue after each job finishes by listening for WorkerMessageHandledEvent in order to send manual notify calls when jobs succeed

We will report jobs that will be retried, because a failed job could indicate a problem even if it eventually succeeds. This is also what we do on other platforms, e.g. Sidekiq in Ruby. The willRetry flag is added as metadata under the Messenger key, so jobs that will be retried can be ignored with a simple callback:

$bugsnag->registerCallback(function (Report $report) {
    $metadata = $report->getMetaData();

    if (isset($metadata['Messenger']['willRetry'])
        && $metadata['Messenger']['willRetry']
    ) {
        return false;
    }
});

Changeset

• add symfony/messenger to CI
• add Symfony 4.0.* to CI matrix (as messenger was added in 4.1)
• add listener for WorkerMessageFailedEvent to notify on failure
• add listener for WorkerMessageHandledEvent to flush on success