Update micrometer auto-configuration so that a `CompositeMeterRegistry`
is only created when more than one `MeterRegistry` bean is declared.
When a composite is crated, it is marked as `@Primary` so that it
can be directly injected. Meter registries can now be defined directly
as beans, and auto-configuration can back off in the usual way.

The `MeterRegistryConfigurer` is now called `MeterRegistryCustomizer`
and is generically types so it's easy to apply customizations to a
particular `MeterRegistry` implementation.

Fixes gh-11799
Co-authored-by: Jon Schneider <jschneider@pivotal.io>