Parsing errors in app/services/metrics_service.rb for Prometheus /-/metrics

There were a couple of prometheus errors first seen in 9.4 RC6:

https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/39196/
https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/39195/

JSON::ParserError
419: unexpected token at '"ra'
app/services/metrics_service.rb in prometheus_metrics_text at line 11
  ].freeze

  def prometheus_metrics_text
    Prometheus::Client::Formats::Text.marshal_multiprocess(multiprocess_metrics_path)
  end

Prometheus::Client::ParsingError
exception undefined method `unpack' for nil:NilClass while processing metrics file /run/gitlab/unicorn/counter_40320.db