Improve Tracing

 - Introduce pattern tracing, that can intercept failed or new coordinations matching various filters
 - Support additional tracing event collection modes (SAMPLE and RING)

patch by Benedict; reviewed by Alex Petrov for CASSANDRA-20911

Author: belliottsmith

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "modules/accord"]
 	path = modules/accord
-	url = https://github.com/apache/cassandra-accord.git
-	branch = trunk
+	url = https://github.com/belliottsmith/cassandra-accord.git
+	branch = improve-tracing

modules/accord

@@ -198,8 +198,6 @@ public String toString()
 
     public static class Listener implements CoordinatorEventListener
     {
-        public static final Listener instance = new Listener();
-
         private AccordCoordinatorMetrics forTransaction(TxnId txnId)
         {
             if (txnId != null)

src/java/org/apache/cassandra/db/virtual/AccordDebugKeyspace.java

@@ -112,8 +112,6 @@ public String toString()
 
     public static class Listener implements ReplicaEventListener
     {
-        public static final Listener instance = new Listener();
-
         private AccordReplicaMetrics forTransaction(TxnId txnId)
         {
             if (txnId != null)

src/java/org/apache/cassandra/metrics/AccordCoordinatorMetrics.java

@@ -132,6 +132,10 @@ static class SnapshotBuilder
         }
     }
 
+    public static void touch()
+    {
+    }
+
     private AccordSystemMetrics()
     {
         Invariants.expect(AccordService.isSetup());

src/java/org/apache/cassandra/metrics/AccordReplicaMetrics.java

@@ -41,6 +41,7 @@
 import accord.api.ConfigurationService.EpochReady;
 import accord.primitives.Txn;
 import org.apache.cassandra.metrics.AccordReplicaMetrics;
+import org.apache.cassandra.metrics.AccordSystemMetrics;
 import org.apache.cassandra.service.accord.api.AccordViolationHandler;
 import org.apache.cassandra.utils.Clock;
 import org.apache.cassandra.utils.concurrent.AsyncFuture;
@@ -307,6 +308,7 @@ public synchronized static AccordService startup(NodeId tcmId)
         instance = as;
 
         AccordReplicaMetrics.touch();
+        AccordSystemMetrics.touch();
         AccordViolationHandler.setup();
 
         WatermarkCollector.fetchAndReportWatermarksAsync(as.configService);