Add thread total started count to SystemPublicMetrics

See gh-4356

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/SystemPublicMetrics.java

@@ -119,6 +119,8 @@ public class SystemPublicMetrics implements PublicMetrics, Ordered {
 				(long) threadMxBean.getPeakThreadCount()));
 		result.add(new Metric<Long>("threads.daemon",
 				(long) threadMxBean.getDaemonThreadCount()));
+		result.add(new Metric<Long>("threads.totalStarted",
+				threadMxBean.getTotalStartedThreadCount()));
 		result.add(new Metric<Long>("threads", (long) threadMxBean.getThreadCount()));
 	}
 

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/SystemPublicMetricsTests.java

@@ -52,6 +52,7 @@ public class SystemPublicMetricsTests {
 
 		assertTrue(results.containsKey("threads.peak"));
 		assertTrue(results.containsKey("threads.daemon"));
+		assertTrue(results.containsKey("threads.totalStarted"));
 		assertTrue(results.containsKey("threads"));
 
 		assertTrue(results.containsKey("classes.loaded"));