Make Datadog Reporter disabled if no apikey configured

2019-10-18T16:03:01,789 ERROR [DatadogReporter] Error reporting metrics to Datadog
java.lang.IllegalArgumentException: Illegal character in query at index 49: https://app.datadoghq.com/api/v1/series?api_key=${datadog.apikey}
	at java.net.URI.create(URI.java:883) ~[?:?]
	at org.apache.http.client.methods.HttpPost.<init>(HttpPost.java:73) ~[httpclient-4.5.6.jar:4.5.6]
	at org.apache.http.client.fluent.Request.Post(Request.java:96) ~[fluent-hc-4.3.2.jar:4.3.2]
	at org.coursera.metrics.datadog.transport.HttpTransport$HttpRequest.send(HttpTransport.java:102) ~[metrics-datadog-1.0.2.jar:?]
	at org.coursera.metrics.datadog.DatadogReporter.report(DatadogReporter.java:96) [metrics-datadog-1.0.2.jar:?]
	at com.codahale.metrics.ScheduledReporter.report(ScheduledReporter.java:237) [metrics-core-4.0.3.jar:4.0.3]
	at com.codahale.metrics.ScheduledReporter.lambda$start$0(ScheduledReporter.java:177) [metrics-core-4.0.3.jar:4.0.3]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) [?:?]
	at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:305) [?:?]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:305) [?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
	at java.lang.Thread.run(Thread.java:834) [?:?]
Caused by: java.net.URISyntaxException: Illegal character in query at index 49: https://app.datadoghq.com/api/v1/series?api_key=${datadog.apikey}
	at java.net.URI$Parser.fail(URI.java:2915) ~[?:?]
	at java.net.URI$Parser.checkChars(URI.java:3086) ~[?:?]
	at java.net.URI$Parser.parseHierarchical(URI.java:3174) ~[?:?]
	at java.net.URI$Parser.parse(URI.java:3116) ~[?:?]
	at java.net.URI.<init>(URI.java:600) ~[?:?]
	at java.net.URI.create(URI.java:881) ~[?:?]
	... 12 more

log.warn("Missing Datadog API key: Datadog Reporter disabled");