Add warning about export of metrics being switched off

E.g. just adding Dropwizard to the classpath switches it off which
might be surprising. We need a @ExportMetricReader to do the export.

spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -1061,7 +1061,6 @@ hooked up to an `Exporter` and fed metric updates every 5 seconds (configured vi
 `spring.metrics.export.delay-millis`). In addition, any `MetricReader` that you define and
 mark as `@ExportMetricReader` will have its values exported by the default exporter.
 
-
 The default exporter is a `MetricCopyExporter` which tries to optimize itself by not
 copying values that haven't changed since it was last called (the optimization can be
 switched off using a flag `spring.metrics.export.send-latest`). Note also that the
@@ -1074,6 +1073,10 @@ values for specific `MetricWriters` can be set as
 `spring.metrics.export.triggers.<name>.*` where `<name>` is a bean name (or pattern for
 matching bean names).
 
+WARNING: The automatic export of metrics is disabled if you switch off the
+default `MetricRepository` (e.g. by using Dropwizard metrics). You can get back the
+same functionality be declaring a bean of your own of type `MetricReader` and 
+declaring it to be `@ExportMetricReader`.
 
 
 [[production-ready-metric-writers-export-to-redis]]