Add assertions to aid troubleshooting Hot Threads test failures (#72837)

Sometimes generating hot threads will fail due to attempting to create a
TimeValue with a negative duration. This commit adds asserts at the time
we get the duration that will be used to create the TimeValue to aid the
debugging process.

server/src/main/java/org/elasticsearch/monitor/jvm/HotThreads.java

@@ -180,11 +180,20 @@ public class HotThreads {
         // skip that for now
         final ToLongFunction<MyThreadInfo> getter;
         if ("cpu".equals(type)) {
-            getter = o -> o.cpuTime;
+            getter = o -> {
+                assert o.cpuTime >= -1 : "cpu time should not be negative, but was " + o.cpuTime + ", thread info: " + o.info;
+                return o.cpuTime;
+            };
         } else if ("wait".equals(type)) {
-            getter = o -> o.waitedTime;
+            getter = o -> {
+                assert o.waitedTime >= -1 : "waited time should not be negative, but was " + o.waitedTime + ", thread info: " + o.info;
+                return o.waitedTime;
+            };
         } else if ("block".equals(type)) {
-            getter = o -> o.blockedTime;
+            getter = o -> {
+                assert o.blockedTime >= -1 : "blocked time should not be negative, but was " + o.blockedTime + ", thread info: " + o.info;
+                return o.blockedTime;
+            };
         } else {
             throw new IllegalArgumentException("expected thread type to be either 'cpu', 'wait', or 'block', but was " + type);
         }