Do not lock on settings keyset if keys initialized (#52435)

Every time a setting#exist call is made we lock on the keyset to ensure
that it has been initialized. This a heavyweight operation that only
should be done once. This commit moves to a volatile read instead to
prevent unnecessary locking.

server/src/main/java/org/elasticsearch/common/settings/Settings.java

@@ -716,7 +716,9 @@ public final class Settings implements ToXContentFragment {
 
     /** Returns the fully qualified setting names contained in this settings object. */
     public Set<String> keySet() {
+        if (keys.get() == null) {
             synchronized (keys) {
+                // Check that the keys are still null now that we have acquired the lock
                 if (keys.get() == null) {
                     if (secureSettings == null) {
                         keys.set(settings.keySet());
@@ -727,6 +729,7 @@ public final class Settings implements ToXContentFragment {
                     }
                 }
             }
+        }
         return keys.get();
     }