16:38:57,964 ERROR [io.undertow.request] (management I/O-1) UT005071: Undertow request failed HttpServerExchange{ GET /metrics/test}: java.lang.IllegalStateException: UT000002: The response has already been started
	at io.undertow.server.HttpServerExchange.setStatusCode(HttpServerExchange.java:1406)
	at org.wildfly.extension.microprofile.metrics.MetricsContextService$1.lambda$handleRequest$0(MetricsContextService.java:85)
	at io.smallrye.metrics.MetricsRequestHandler.handleRequest(MetricsRequestHandler.java:142)
	at io.smallrye.metrics.MetricsRequestHandler.handleRequest(MetricsRequestHandler.java:73)
	at org.wildfly.extension.microprofile.metrics.MetricsContextService$1.handleRequest(MetricsContextService.java:84)
	at org.jboss.as.domain.http.server.security.RealmReadinessHandler.handleRequest(RealmReadinessHandler.java:51)
	at org.jboss.as.domain.http.server.security.ServerErrorReadinessHandler.handleRequest(ServerErrorReadinessHandler.java:35)
	at io.undertow.server.handlers.PathHandler.handleRequest(PathHandler.java:91)
	at io.undertow.server.handlers.ChannelUpgradeHandler.handleRequest(ChannelUpgradeHandler.java:211)
	at io.undertow.server.handlers.cache.CacheHandler.handleRequest(CacheHandler.java:92)
	at io.undertow.server.handlers.error.SimpleErrorPageHandler.handleRequest(SimpleErrorPageHandler.java:78)
	at io.undertow.server.handlers.CanonicalPathHandler.handleRequest(CanonicalPathHandler.java:49)
	at org.jboss.as.domain.http.server.ManagementHttpRequestHandler.handleRequest(ManagementHttpRequestHandler.java:57)
	at org.jboss.as.domain.http.server.cors.CorsHttpHandler.handleRequest(CorsHttpHandler.java:75)
	at org.jboss.as.domain.http.server.ManagementHttpServer$UpgradeFixHandler.handleRequest(ManagementHttpServer.java:664)
	at io.undertow.server.Connectors.executeRootHandler(Connectors.java:376)
	at io.undertow.server.protocol.http.HttpReadListener.handleEventWithNoRunningRequest(HttpReadListener.java:255)
	at io.undertow.server.protocol.http.HttpReadListener.handleEvent(HttpReadListener.java:136)
	at io.undertow.server.protocol.http.HttpOpenListener.handleEvent(HttpOpenListener.java:162)
	at io.undertow.server.protocol.http.HttpOpenListener.handleEvent(HttpOpenListener.java:100)
	at io.undertow.server.protocol.http.HttpOpenListener.handleEvent(HttpOpenListener.java:57)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at org.xnio.ChannelListeners$10.handleEvent(ChannelListeners.java:291)
	at org.xnio.ChannelListeners$10.handleEvent(ChannelListeners.java:286)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at org.xnio.nio.QueuedNioTcpServer$1.run(QueuedNioTcpServer.java:132)
	at org.xnio.nio.WorkerThread.safeRun(WorkerThread.java:612)
	at org.xnio.nio.WorkerThread.run(WorkerThread.java:479)

$ curl -v http://localhost:9990/metrics/test
*   Trying ::1:9990...
* TCP_NODELAY set
* connect to ::1 port 9990 failed: Connection refused
*   Trying 127.0.0.1:9990...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 9990 (#0)
> GET /metrics/test HTTP/1.1
> Host: localhost:9990
> User-Agent: curl/7.65.3
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 404 Not Found
< Connection: keep-alive
< Content-Length: 25
< Date: Thu, 22 Aug 2019 14:57:20 GMT
< 
* Connection #0 to host localhost left intact
Bad scope requested: test

The 404 Not Found HTTP response code is expected and also 'Bad scope request: test' in this case. Although, I think that mentioned IllegalStateException should be avoided. Also, this exception is not thrown in WildFly 17.0.1.