Allow urlText to be saved on metric images
What does this MR do and why?
This adds the ability to add a Url Text (text describing what the image or external URL is, i.e 'Production error rate').
It also adds the ability to update both the url and url_text fields on the metric image.
The backend for this change was added in !78430 (merged).
Screenshots or screen recordings
| header | header | 
|---|---|
| Form |  | 
| URL with URL Text |  | 
How to set up and validate locally
- Ensure you have at least a GitLab Premium account.
- Head to Monitor -> Incidents and create an incident.
- Once created, add a metric via the metric tab.
- Add / Edit / Delete as you please.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
- 
I have evaluated the MR acceptance checklist for this MR. 
Related to #346171 (closed)
Edited  by Sean Arnold
