Symfony version(s) affected: 4.4

Description
Messenger config options array merged wrong. Values inside specific environment config of options array are merged to common config with numbers as keys.

How to reproduce

1. Create messenger.yaml in config/packages/ with following contents:

framework:
    messenger:
        transports:
            async: "%env(MESSENGER_TRANSPORT_DSN)%"
            options:
                auto_setup: false

1. Then, create another, for example, for the test environment config/packages/test/ and add options there:

framework:
    messenger:
        transports:
            async: "%env(MESSENGER_TRANSPORT_DSN)%"
            options:
                wait_time: 60

1. Run console debug:config:

console debug:config framework

#...
messenger:
    transports:
        async:
            dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
            options:
                auto_setup: true
#...

1. Run console debug:config for test-environment:

APP_ENV=test console debug:config framework

Output for test-environment:

#...
messenger:
    transports:
        async:
            dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
            options:
                auto_setup: true
                0: 60
#...

Additional context

That's happened because in ConfigTree options are defined as arrayNode and has a prototype('variable'). So for combining config together PrototypedArrayNode::mergeValues() would be called. And there is an issue. For those PrototypedArrayNode that have not $keyAttribute $rightSide values added without keys:

foreach ($rightSide as $k => $v) {
    // prototype, and key is irrelevant, append the element
    if (null === $this->keyAttribute) {
        $leftSide[] = $v;
        continue;
    }

Possible soultion
So the solution is to add useAttributeAsKey inside ConfigTree for arrayNode('options'). PR will be submitted soon