Fix custom metrics dashboard when max_value is set

What does this MR do?

Resolves #214360 (closed)

Dashboard failed to load, because new attribute max_value added to panel wasn't added to https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/performance_monitoring/prometheus_panel.rb which caused errors.

In order to address that I've added missing attribute and updated specs. Also to prevent such regression from happening in the future I've added error handling to dashboard processing stage, to allow it to fail in less impactful way. In the end I've added failing stage PanelIdInserter into https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/metrics/dashboard/processor_spec.rb to improve integration testing, which should also help detecting similar regression in the future.

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