Minor logging improvements (#38084)

Fixes some log messages that caused some minor confusion when digging through a
log generated by a failing test.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/cluster/coordination/Coordinator.java

@@ -557,8 +557,13 @@ void becomeFollower(String method, DiscoveryNode leaderNode) {
         assert Thread.holdsLock(mutex) : "Coordinator mutex not held";
         assert leaderNode.isMasterNode() : leaderNode + " became a leader but is not master-eligible";
 
-        logger.debug("{}: coordinator becoming FOLLOWER of [{}] in term {} (was {}, lastKnownLeader was [{}])",
-            method, leaderNode, getCurrentTerm(), mode, lastKnownLeader);
+        if (mode == Mode.FOLLOWER && Optional.of(leaderNode).equals(lastKnownLeader)) {
+            logger.trace("{}: coordinator remaining FOLLOWER of [{}] in term {}",
+                method, leaderNode, getCurrentTerm());
+        } else {
+            logger.debug("{}: coordinator becoming FOLLOWER of [{}] in term {} (was {}, lastKnownLeader was [{}])",
+                method, leaderNode, getCurrentTerm(), mode, lastKnownLeader);
+        }
 
         final boolean restartLeaderChecker = (mode == Mode.FOLLOWER && Optional.of(leaderNode).equals(lastKnownLeader)) == false;
 

server/src/main/java/org/elasticsearch/cluster/coordination/FollowersChecker.java

@@ -374,9 +374,10 @@ void failNode(String reason) {
                 public void run() {
                     synchronized (mutex) {
                         if (running() == false) {
-                            logger.debug("{} condition no longer applies, not marking faulty", discoveryNode);
+                            logger.trace("{} no longer running, not marking faulty", FollowerChecker.this);
                             return;
                         }
+                        logger.debug("{} marking node as faulty", FollowerChecker.this);
                         faultyNodes.add(discoveryNode);
                         followerCheckers.remove(discoveryNode);
                     }