Possibly malformed event payload

Hiya,

I'm running into a sporadic issue with a custom Python plugin. Occasionally, it looks like the plugin server receives a malformed event to handle.

Kong Gateway: 3.3.0
Python-pdk: 0.36
Kong Ingress Controller: 2.10
Proxy env vars:
```
...
KONG_PLUGINS : bundled,my-plugin
KONG_PLUGINSERVER_MY_PLUGIN_QUERY_CMD : /home/kong/plugins/python/my_plugin.py -p /kong-prefix/ -d
KONG_PLUGINSERVER_MY_PLUGIN_SOCKET : /kong-prefix/my-plugin.sock
KONG_PLUGINSERVER_MY_PLUGIN_START_CMD : /home/kong/plugins/python/my_plugin.py -p /kong-prefix/ --no-lua-style -g
KONG_PREFIX : /kong-prefix/
...
```
Logs:
```
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] INFO - [02:08:35] instance #1259 of my-plugin started, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] INFO - [02:08:35] instance #1260 of my-plugin started, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] ERROR - [02:08:35] rpc: #6285 exception: Traceback (most recent call last):, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] File "/usr/local/lib/python3.10/dist-packages/kong_pdk/listener.py", line 68, in handle, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] ret = getattr(self.ps, cmd_r)(*args), context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] File "/usr/local/lib/python3.10/dist-packages/kong_pdk/server.py", line 54, in wrapper, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] raise ex, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] File "/usr/local/lib/python3.10/dist-packages/kong_pdk/server.py", line 51, in wrapper, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] r = fn(*args, **kwargs), context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] File "/usr/local/lib/python3.10/dist-packages/kong_pdk/server.py", line 278, in handle_event, context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] iid = event['InstanceId'], context: ngx.timer
2023/10/26 18:08:35 [info] 2375#0: *311 [my-plugin:2377] KeyError: 'InstanceId', context: ngx.timer
2023/10/26 18:08:35 [error] 2375#0: *188104 [kong] mp_rpc.lua:354 [my-plugin] 'InstanceId', client: 10.0.131.51, server: kong, request: "GET /foo HTTP/1.1", host: "bar"
```

The `KeyError: 'InstanceId'` makes me think that the error is from https://github.com/Kong/kong-python-pdk/blob/master/kong_pdk/server.py#L278, and that the `event` argument is sometimes malformed.

Any ideas? Please and thank you!