Unable to omit admin ui in prod due to metricSummary

[Conor-Burns]

Describe the bug
We build a docker image for the vendure backend and deploy the admin UI as a static app on cloudflare pages. When omitting the AdminUiPlugin from the config we get a clean docker image but no metric in the ui as the plugin isn't loaded. The graphql extensions for the metrics seem to come directly from the AdminUiPlugin, which isn't ideal for a setup like this.

To Reproduce

1. omit AdminUiPlugin
2. deploy static admin UI
3. add metrics
4. Error: Unknown type "MetricType". Cannot query field "metricSummary" on type "Query".

Expected behavior
being able to add AdminUiPlugin without any config to just load it
Environment (please complete the following information):

• @vendure/core version: 2.0.2
• Nodejs version 18 lts
• Database (mysql/postgres etc): postgres

Additional context
In the mean time we bundle the UI also in the backend

[michaelbromley]

I'm going to make it so you can add just AdminUiPlugin without the init() call, and it will just expose the metrics query without then serving the Admin UI app.