Add read API for Alert Metric Images

What does this MR do and why?

API Request & responses

View images

Curl

curl --location --request GET 'http://gdk.test:3000/api/v4/projects/6/alert_management_alerts/12/metric_images' \
--header 'PRIVATE-TOKEN: glpat-DEpkrRZUMVgm6ixjRGCr'

Response

[
    {
        "id": 2,
        "created_at": "2022-01-27T02:17:39.419Z",
        "filename": "s7mnOhs.jpg",
        "file_path": "http://gdk.test:3000/uploads/-/system/alert_management_metric_image/file/2/s7mnOhs.jpg",
        "url": "http://gitlab.com",
        "url_text": "Grafana metric"
    },
    {
        "id": 3,
        "created_at": "2022-01-27T02:18:24.055Z",
        "filename": "s7mnOhs.jpg",
        "file_path": "http://gdk.test:3000/uploads/-/system/alert_management_metric_image/file/3/s7mnOhs.jpg",
        "url": "http://gitlab.com",
        "url_text": "Grafana metric"
    }
]

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

First create an alert:

1. Navigate to Settings > Monitor > Alerts > Current integrations
2. Create & enable a new HTTP Endpoint alert integration if none exist
3. For an existing integration, click > Send test alert to trigger a test with a sample payload, like this one:
   • {"title":"Triggering a test alert to associate with an incident"}

Then, create a metric image:

file = Rack::Test::UploadedFile.new('spec/fixtures/rails_sample.jpg', 'image/jpg')

alert = AlertManagement::Alert.last

alert.metric_images.create(file: file, url: 'http://www.gitlab.com', url_text: 'Example metric')

Then call the API using the example above.

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 #340852 (closed)