Confirm WebServerFactoryCustomizer's execution order

This commit confirms WebServerFactoryCustomizer's execution precedes other bean creations.

See micrometer-metrics/micrometer#911

src/main/java/com/izeye/sample/config/JettyConfig.java

@@ -0,0 +1,37 @@
+package com.izeye.sample.config;
+
+import org.springframework.boot.web.embedded.jetty.ConfigurableJettyWebServerFactory;
+import org.springframework.boot.web.server.WebServerFactoryCustomizer;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import org.eclipse.jetty.server.Server;
+import org.eclipse.jetty.util.thread.ThreadPool;
+
+/**
+ * Configuration for Jetty.
+ *
+ * @author Johnny Lim
+ */
+@Configuration
+public class JettyConfig {
+
+	private volatile Server server;
+
+	@Bean
+	public WebServerFactoryCustomizer<ConfigurableJettyWebServerFactory> jettyCustomizer() {
+		return (jetty) -> {
+			jetty.addServerCustomizers(this::setServer);
+		};
+	}
+
+	@Bean
+	public ThreadPool jettyThreadPool() {
+		return this.server.getThreadPool();
+	}
+
+	private void setServer(Server server) {
+		this.server = server;
+	}
+
+}