Add StateTransitionReason to ec2:DescribeInstances response (#7495)

moto/ec2/responses/instances.py

@@ -463,7 +463,7 @@ INSTANCE_TEMPLATE = """<item>
           <privateDnsName>{{ instance.private_dns }}</privateDnsName>
           <publicDnsName>{{ instance.public_dns }}</publicDnsName>
           <dnsName>{{ instance.public_dns }}</dnsName>
-          <reason/>
+          <reason>{{ instance._reason }}</reason>
           {% if instance.key_name is not none %}
              <keyName>{{ instance.key_name }}</keyName>
           {% endif %}

tests/test_ec2/test_instances.py

@@ -973,6 +973,7 @@ def test_instance_start_and_stop():
 
     instance1.reload()
     assert instance1.state == {"Code": 80, "Name": "stopped"}
+    assert instance1.state_transition_reason.startswith("User initiated")
 
     with pytest.raises(ClientError) as ex:
         client.start_instances(InstanceIds=[instance1.id], DryRun=True)