Add pending migrations to mr metric man

Description

This MR adds a new section to mr metric man to output pending migrations. It prints out a line for each migration containing:

• name (hyperlinked to the API endpoint to give you more details)
• status (in progress, halted, or pending)
• started at

Example output

• 🔎 pending migrations

<https://gitlab.com/api/v4/admin/search/migrations/20230321154100|BackfillHashedRootNamespaceIdOnUsers>         Status: in progress             Started at: 2023-03-23T15:23:40.335Z
<https://gitlab.com/api/v4/admin/search/migrations/20230411064300|BackfillProjectPermissionsInBlobs>            Status: in progress             Started at: 2023-05-02T19:04:51.672Z