Spring Boot’s metrics require all values to be Numbers. A Dropwizard
Gauge can have a non-Number value. Previously, to prevent this causing
a problem, MetricRegistryMetricReader would check the value of a Gauge
when it’s being added and ignore it if it had a non-Number value.
Unfortunately, retrieving the value of a Gauge can take a non-trivial
amount of time (hence CachedGauge) so this approach, while functional,
could be improved.

This commit updates the filtering to happen when a Metric is being
retrieved from MetricRegistryMetricReader (via findOne or findAll)
when its value is required anyway. At this point, any Gauge with a
non-Number value is ignored.

Closes gh-4874