400 response when loading due to missing "tags" registry

[w33ble]

Kibana version: 6.6+

Describe the bug:

When loading Canvas (or Kibana in general, thanks to #27175) and the socket is started, there's a lone 400 response related to a "tags" registry.

Not sure if this was introduced as part of #26068 or if the issue existed before it. I also couldn't tell at a quick glance why it's trying to load tags.

Steps to reproduce:

1. Open debugger console
2. Load Kibana (or Canvas once [Regression] Socket connection is always created #27175 is fixed)

Screenshots (if relevant):

GET http://localhost:5601/ztz/api/canvas/plugins?type=tags net::ERR_ABORTED 400 (Bad Request)

[elasticmachine]

Pinging @elastic/kibana-canvas

[elasticmachine]

Pinging @elastic/kibana-app

[w33ble]

I believe this is what's causing the error, but I haven't dug into why the registry isn't exposed via the endpoint.