Change the way the Doctrine tracing middleware is registered in DBAL >=3.x

[ste93cry]

Fixes #805. The problem that was reported is that the MiddlewarePass compiler pass in DoctrineBundle adds a new call to setMiddlewares(), which means that all the middlewares added before get lost. Technically speaking, if you load SentryBundle after DoctrineBundle, it works as expected, because our code appends our middleware to the existing list instead of replacing it entirely. However, as noted by @fridtjof, a better integration approach is to tag our middleware as doctrine.middleware and let the Doctrine bundle do all the rest of the work to properly configure the connection.

[fridtjof]

Do note that this raises the required doctrine-bundle version to 2.6 (as opposed to the current ^2.5 listed in require-dev), as their tagged middleware support was only introduced then: https://github.com/doctrine/DoctrineBundle/releases/tag/2.6.0, doctrine/DoctrineBundle#1472

[Jean85]

This is a very interesting approach, and also forward compatible, thank you!

PS: do we have any E2E test to check if all of this is working?

[ste93cry]

Do note that this raises the required doctrine-bundle version to 2.6

Nice catch. Unfortunately, I realized that these changes are not compatible with version 1.12 of the bundle, which we still support. It seems that we declare the compatibility with that version mainly because of Symfony 3.4, but since we nowadays require at least version 4.4 of the framework, I think we can just drop the support for the the older 1.x...

do we have any E2E test to check if all of this is working?

No, we don't. There are some E2E tests for other stuffs, but nothing specifically tailored at asserting that the tracing integrations work as expected. It's something we could definitely improve in the future though...