HBASE-23324 Deprecate clients that connect to Zookeeper (#5745)

Our objective is to remove ZooKeeper from our public interface, remaking it as an internal
concern. Connecting to a cluster via ZooKeeper quorum will be considered deprecated starting in
2.6. Our default connection mechanism will switch to via RPC in 3.0 And finally we intend to
remove the ZooKeeper connection mechanism from client-facing APIs in 4.0.

Signed-off-by: Bryan Beaudreault <bbeaudreault@apache.org>

hbase-client/src/main/java/org/apache/hadoop/hbase/client/ZKConnectionRegistry.java

@@ -33,6 +33,7 @@
 import org.apache.commons.lang3.mutable.MutableInt;
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.hbase.ClusterId;
+import org.apache.hadoop.hbase.HBaseInterfaceAudience;
 import org.apache.hadoop.hbase.HRegionLocation;
 import org.apache.hadoop.hbase.RegionLocations;
 import org.apache.hadoop.hbase.ServerName;
@@ -51,20 +52,37 @@
 
 /**
  * Zookeeper based registry implementation.
+ * @deprecated As of 2.6.0, replaced by {@link RpcConnectionRegistry}, which is the default
+ *             connection mechanism as of 3.0.0. Expected to be removed in 4.0.0.
+ * @see <a href="https://issues.apache.org/jira/browse/HBASE-23324">HBASE-23324</a> and its parent
+ *      ticket for details.
  */
-@InterfaceAudience.Private
+@Deprecated
+@InterfaceAudience.LimitedPrivate(HBaseInterfaceAudience.CONFIG)
 class ZKConnectionRegistry implements ConnectionRegistry {
 
   private static final Logger LOG = LoggerFactory.getLogger(ZKConnectionRegistry.class);
 
+  private static final Object WARN_LOCK = new Object();
+  private static volatile boolean NEEDS_LOG_WARN = true;
+
   private final ReadOnlyZKClient zk;
 
   private final ZNodePaths znodePaths;
 
   // User not used, but for rpc based registry we need it
-  ZKConnectionRegistry(Configuration conf, User user) {
+  ZKConnectionRegistry(Configuration conf, User ignored) {
     this.znodePaths = new ZNodePaths(conf);
     this.zk = new ReadOnlyZKClient(conf);
+    if (NEEDS_LOG_WARN) {
+      synchronized (WARN_LOCK) {
+        if (NEEDS_LOG_WARN) {
+          LOG.warn(
+            "ZKConnectionRegistry is deprecated. See https://hbase.apache.org/book.html#client.rpcconnectionregistry");
+          NEEDS_LOG_WARN = false;
+        }
+      }
+    }
   }
 
   private interface Converter<T> {