Add new table to connect issues to metrics

What does this MR do and why?

• Adds a new table observability_metrics_issues_connections to store the connection between an Issue and a Metric where metrics are stored in Gitlab Observability Backend.
  • This MR doesn't actually connect to GOB at all, so doesn't require it to validate the MR.
• The table also includes a namespace_id sharding key for cells which is automatically populated using a Rails after_create callback.

MR acceptance checklist

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How to set up and validate locally

1. Run the migrations in this MR (rails db:migrate)

2. Open a rails console.

3. Create a new instance of the model, with an issue and save it.

   a = Observability::MetricsIssuesConnection.new(issue: Issue.last, metric_type: :sum_type, metric_name: "testing1234")
   a.save!

4. Note the additional UPDATE statement after the object is persisted that should set namespace_id.

Related to gitlab-org/opstrace/opstrace#2884 (closed)