Stop disabling metrics export auto-configuration when related enable flag is disabled

[snicoll]

Relates to #13408

We also have one enabled flag per monitoring system we support. This one is different as there is a direct counter part in micrometer itself.

Right now, we back-off if the flag is set to false which means that we never effectively set the Micrometer flag to false (we just don't auto-configure anything). Given the scope of this flag, we should probably remove the condition.

/cc @jkschneider

[snicoll]

We've discussed this one on the team call and we're not sure what to do really. We should spend more time on this and perhaps introduce a more generic/friendly way of disabling auto-configurations without a link to the class itself.

[snicoll]

We've discussed this one and we're considering adding a convenient way of disabling auto-configurations. One idea was a way to name auto-configurations and have an automatic property being generated for each to be able to disable it.

There is also a slightly related conversation where auto-configurations could be grouped in "themes" and we could apply the same mechanism to easily exclude them.

We've decided to park this one for now.