ScheduledReporter not properly closed on shutdown

[snicoll]

This is similar to #108 and should probably be moved to the relevant project.

Reproducer: https://github.com/snicoll/demo-tanzu-observability

Steps:

• Start the two apps
• Modify something in dashboard to trigger a restart from devtools

One minute or so after the restart you get this:

java.io.IOException: attempt to send using closed sender
	at com.wavefront.sdk.common.clients.WavefrontClient.sendMetric(WavefrontClient.java:389) ~[wavefront-sdk-java-2.6.4.jar:na]
	at com.wavefront.sdk.entities.metrics.WavefrontMetricSender.sendDeltaCounter(WavefrontMetricSender.java:89) ~[wavefront-sdk-java-2.6.4.jar:na]
	at com.wavefront.internal.reporter.WavefrontInternalReporter.reportCounter(WavefrontInternalReporter.java:422) ~[wavefront-internal-reporter-java-1.7.5.jar:na]
	at com.wavefront.internal.reporter.WavefrontInternalReporter.access$300(WavefrontInternalReporter.java:65) ~[wavefront-internal-reporter-java-1.7.5.jar:na]
	at com.wavefront.internal.reporter.WavefrontInternalReporter$1.report(WavefrontInternalReporter.java:270) ~[wavefront-internal-reporter-java-1.7.5.jar:na]
	at com.wavefront.internal_reporter_java.io.dropwizard.metrics5.ScheduledReporter.report(ScheduledReporter.java:237) ~[wavefront-internal-reporter-java-1.7.5.jar:na]
	at com.wavefront.internal_reporter_java.io.dropwizard.metrics5.ScheduledReporter.lambda$start$0(ScheduledReporter.java:177) ~[wavefront-internal-reporter-java-1.7.5.jar:na]
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) ~[na:na]
	at java.base/java.util.concurrent.FutureTask.runAndReset(FutureTask.java:305) ~[na:na]
	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:305) ~[na:na]
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[na:na]
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[na:na]
	at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]