[metal] Manage staging buffer refcount in command buffer lifetime

This avoids the potential refcount mismatches due to that we can
have command buffers created but never submitted and such.

experimental/metal/direct_command_buffer.m

diff --git a/experimental/metal/direct_command_buffer.m b/experimental/metal/direct_command_buffer.m
index 1835259..45f1448 100644
--- a/experimental/metal/direct_command_buffer.m
+++ b/experimental/metal/direct_command_buffer.m
@@ -381,6 +381,10 @@
 
   *out_command_buffer = &command_buffer->base;
 
+  // Increase command buffer refcount in the shared staging buffer. We tie this to the command
+  // buffer's lifetime to avoid resource leak.
+  iree_hal_metal_staging_buffer_increase_refcount(staging_buffer);
+
   IREE_TRACE_ZONE_END(z0);
   return status;
 }
@@ -392,6 +396,10 @@
 
   iree_hal_metal_command_buffer_reset(command_buffer);
 
+  // Decrease command buffer refcount in the shared staging buffer, and potentially reclaim
+  // resources. We tie this to the command buffer's lifetime to avoid resource leak.
+  iree_hal_metal_staging_buffer_decrease_refcount(command_buffer->staging_buffer);
+
   [command_buffer->state.encoder_event release];  // -1
   IREE_ASSERT_EQ(command_buffer->state.compute_encoder, nil);
   IREE_ASSERT_EQ(command_buffer->state.blit_encoder, nil);