UTF-8: Implement quoting and escaping in UI autocompletion

### Proposal

as part of https://github.com/prometheus/prometheus/issues/15202, we will need to update the autocomplete engine in the UI to know when escaping or quoting is necessary. @juliusv felt this was a big enough subtask to call out separately.