Add url_text column to issuable metric images, and allow updates to url and url_text

What does this MR do and why?

This MR does two things:

1. It adds url_text column to issuable_metric_images, with a limit of 128 chars
2. It adds an update API, for updating the url and url_text fields.

Migration

Up:

== 20220118015633 AddUrlTextToIssuableMetricImages: migrating =================
-- add_column(:issuable_metric_images, :url_text, :text)
   -> 0.0051s
== 20220118015633 AddUrlTextToIssuableMetricImages: migrated (0.0052s) ========

== 20220118020026 AddUrlTextLimitToIssuableMetricImages: migrating ============
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0003s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE issuable_metric_images\nADD CONSTRAINT check_3bc6d47661\nCHECK ( char_length(url_text) <= 128 )\nNOT VALID;\n")
   -> 0.0039s
-- current_schema()
   -> 0.0004s
-- execute("SET statement_timeout TO 0")
   -> 0.0011s
-- execute("ALTER TABLE issuable_metric_images VALIDATE CONSTRAINT check_3bc6d47661;")
   -> 0.0017s
-- execute("RESET statement_timeout")
   -> 0.0017s
== 20220118020026 AddUrlTextLimitToIssuableMetricImages: migrated (0.0291s) ===

Down:

== 20220118020026 AddUrlTextLimitToIssuableMetricImages: reverting ============
-- transaction_open?()
   -> 0.0000s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE issuable_metric_images\nDROP CONSTRAINT IF EXISTS check_3bc6d47661\n")
   -> 0.0091s
== 20220118020026 AddUrlTextLimitToIssuableMetricImages: reverted (0.0378s) ===

== 20220118015633 AddUrlTextToIssuableMetricImages: reverting =================
-- remove_column(:issuable_metric_images, :url_text, :text)
   -> 0.0044s
== 20220118015633 AddUrlTextToIssuableMetricImages: reverted (0.0093s) ========

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

1. Create an Incident via Monitor -> Incidents
2. Upload a metric image via the UI or API: https://docs.gitlab.com/ee/api/issues.html#upload-metric-image
3. Edit the URL and URL Text via the API:

curl --header "PRIVATE-TOKEN: <token>" --form 'file=@<path>' \ --form 'url=http://example.com' --form 'url_text=Example website' "http://gdk.test:3000/api/v4/projects/33/issues/1/metric_images"

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)