Previously, only invocations of /metricName/ would honour the enabled
property and return a not found (404) response. For endpoints which
support nested paths, access to /metricName/foo would ignore the enabled
flag and return an OK (200) response. Furthermore, there was a comment
in EndpointMvcAdapter that suggested that an endpoint shouldn’t be
called when it is disabled, however this was not the case.

This commit updates EndpointWebMvcAutoConfiguration and
JolokiaAutoConfiguration to only register their MvcEndpoint beans if
the underlying endpoint is enabled. This means that an
EndpointMvcAdapter should not be called if its delegate is disabled,
making the comment described above accurate.

The check for the delegate being enabled has been retained so as not to
rely upon the auto-configurations’ behaviour. The methods which handle
nested paths (MetricsMvcEndpoint.value() and
EnvironmentMvcEndpoint.value()) have been updated to add the same check
for the enablement of their delegate.

Fixes gh-2767