ConnectionPool metrics are not exposed with `r2dbc:pool:` based URLs

[snicoll]

See spring-attic/spring-boot-r2dbc#80 for a description of the problem.

It looks like that when a ConnectionFactory is exposed as a @Bean (while being a ConnectionPool effectively, @ConditionalOnBean(ConnectionPool.class) does not match. I don't know if that's to be expected but that vaguely rings a bell.

We could relax the condition and then only handles ConnectionPool in the current registration code.

[snicoll]

I have a "fix" for this problem but it feels a little odd that we have to do that, see snicoll@3c88e65