Fix NPE in ChannelPool metrics methods when pool is uninitialized

When the initial TCP connection to a node fails, ChannelPool.connectFuture
completes (so the pool is added to PoolManager.pools) while `channels` is
still null — initialize() is only called on the success path.

The four methods size(), getAvailableIds(), getInFlight(), and
getOrphanedIds() called Arrays.stream(channels) without guarding against
this, throwing NullPointerException whenever a Dropwizard Metrics reporter
(JMX, Graphite, etc.) scraped the gauge values during the reconnection
window.

Fix: mirror the pattern already used by next(), which checks
singleThreaded.initialized before touching channels. Using the volatile
initialized flag is correct by the JMM: the volatile write at line 365
(initialized = true) happens-after all channels writes, so any thread
that reads initialized == true is guaranteed to see channels as
fully populated.

Fixes: CUSTOMER-413

Author: nikagra

core/src/main/java/com/datastax/oss/driver/internal/core/pool/ChannelPool.java

@@ -210,11 +210,17 @@ public DriverChannel next(@Nullable Token routingKey, @Nullable Integer shardSug
 
   /** @return the number of active channels in the pool. */
   public int size() {
+    if (!singleThreaded.initialized) {
+      return 0;
+    }
     return Arrays.stream(channels).mapToInt(ChannelSet::size).sum();
   }
 
   /** @return the number of available stream ids on all channels in the pool. */
   public int getAvailableIds() {
+    if (!singleThreaded.initialized) {
+      return 0;
+    }
     return Arrays.stream(channels).mapToInt(ChannelSet::getAvailableIds).sum();
   }
 
@@ -223,6 +229,9 @@ public int getAvailableIds() {
    *     {@link #getOrphanedIds() orphaned ids}).
    */
   public int getInFlight() {
+    if (!singleThreaded.initialized) {
+      return 0;
+    }
     return Arrays.stream(channels).mapToInt(ChannelSet::getInFlight).sum();
   }
 
@@ -232,6 +241,9 @@ public int getInFlight() {
    *     might still come from the server.
    */
   public int getOrphanedIds() {
+    if (!singleThreaded.initialized) {
+      return 0;
+    }
     return Arrays.stream(channels).mapToInt(ChannelSet::getOrphanedIds).sum();
   }