Print warning message when connection running out

I believe when connection pool running out of available
resources, there will be all kinds of weird consequences
following. So let's print warning logs when we are going
to waiting indefinitely for the lock

Change-Id: I5ca930cada523bc77c644703a0f02b9160816231
Related-bug: #1871813

oslo_messaging/_drivers/pool.py

@@ -100,6 +100,10 @@ class Pool(object):
                     self._current_size += 1
                     break
 
+                LOG.warning("Connection pool limit exceeded: "
+                            "current size %s surpasses max "
+                            "configured rpc_conn_pool_size %s",
+                            self._current_size, self._max_size)
                 wait_condition(self._cond)
 
         # We've grabbed a slot and dropped the lock, now do the creation