Update docs for new metrics and some existing ones. Add test

Author: abhishekrb19

docs/operations/metrics.md

@@ -149,10 +149,13 @@ to represent the task ID are deprecated and will be removed in a future release.
 |Metric|Description|Normal value|
 |------|-----------|------------|
 |`jetty/numOpenConnections`|Number of open jetty connections.|Not much higher than number of jetty threads.|
-|`jetty/threadPool/total`|Number of total workable threads allocated.|The number should equal to `threadPoolNumIdleThreads` + `threadPoolNumBusyThreads`.|
-|`jetty/threadPool/idle`|Number of idle threads.|Less than or equal to `threadPoolNumTotalThreads`. Non zero number means there is less work to do than configured capacity.|
-|`jetty/threadPool/busy`|Number of busy threads that has work to do from the worker queue.|Less than or equal to `threadPoolNumTotalThreads`.|
-|`jetty/threadPool/isLowOnThreads`|A rough indicator of whether number of total workable threads allocated is enough to handle the works in the work queue.|0|
+|`jetty/threadPool/total`|Number of total workable threads allocated. This includes internal threads and threads that are ready to serve requests.|Equals the total number of usable threads in the Jetty thread pool (idle + busy).|
+|`jetty/threadPool/ready`| Number of threads that are ready to serve requests. | Equals the total number of usable threads in the Jetty thread pool to serve requests (idle + busy).|
+|`jetty/threadPool/utilized`| Number of threads currently in use by the thread pool to serve requests. | Typically well below ready.|
+|`jetty/threadPool/utilizationRate`| Fraction of thread pool capacity currently in use to serve requests. | A value of 0.0 means that the thread pool is not utilized, while a value of 1.0 means that the thread pool is fully utilized.|
+|`jetty/threadPool/idle`|Number of idle threads.|Less than or equal to the total number of usable threads. A non-zero value means there is less work to do than configured capacity.|
+|`jetty/threadPool/busy`|Number of busy threads that have work to do from the worker queue.|Less than or equal to the total number of usable threads.|
+|`jetty/threadPool/isLowOnThreads`|A rough indicator of whether the number of total workable threads allocated is enough to handle the work in the work queue.|0|
 |`jetty/threadPool/min`|Number of minimum threads allocatable.|`druid.server.http.numThreads` plus a small fixed number of threads allocated for Jetty acceptors and selectors.|
 |`jetty/threadPool/max`|Number of maximum threads allocatable.|`druid.server.http.numThreads` plus a small fixed number of threads allocated for Jetty acceptors and selectors.|
 |`jetty/threadPool/queueSize`|Size of the worker queue.|Not much higher than `druid.server.http.queueSize`.|

server/src/test/java/org/apache/druid/server/initialization/jetty/JettyServerModuleTest.java

@@ -53,6 +53,9 @@ public void testJettyServerModule()
     Mockito.when(jettyServerThreadPool.getMaxThreads()).thenReturn(100);
     Mockito.when(jettyServerThreadPool.getQueueSize()).thenReturn(50);
     Mockito.when(jettyServerThreadPool.getBusyThreads()).thenReturn(60);
+    Mockito.when(jettyServerThreadPool.getReadyThreads()).thenReturn(20);
+    Mockito.when(jettyServerThreadPool.getUtilizedThreads()).thenReturn(5);
+    Mockito.when(jettyServerThreadPool.getUtilizationRate()).thenReturn(0.45);
 
     JettyServerModule.JettyMonitor jettyMonitor = new JettyServerModule.JettyMonitor();
 
@@ -67,6 +70,9 @@ public void testJettyServerModule()
     serviceEmitter.verifyValue("jetty/threadPool/max", 100);
     serviceEmitter.verifyValue("jetty/threadPool/queueSize", 50);
     serviceEmitter.verifyValue("jetty/threadPool/busy", 60);
+    serviceEmitter.verifyValue("jetty/threadPool/ready", 20);
+    serviceEmitter.verifyValue("jetty/threadPool/utilized", 5);
+    serviceEmitter.verifyValue("jetty/threadPool/utilizationRate", 0.45);
   }
 
   @Test