Previously, MetricExportAutoConfiguration consumed
ExportMetricWriter-annotated GaugeWriter beans in its constructor and
also produced such a bean from one of its @Bean methods. This cycle
caused a BeanCurrentlyInCreationException to be thrown when the
bean method was active (the spring.metrics.export.statsd.host property
was set).

This commit break the cycle by moving the bean method into a separate,
nested configuration class. It also updates the existing test for
auto-configuration of a Statsd writer to catch any possible cycles
and to verify that the writer has be registered with the
MetricsExporter.

Closes gh-6544