Extract watchdog event reporter

What does this MR do and why?

It extracts metric and logs to eventreporter.

Event reporter is configurable, and it allows us to use separate event reporter for sidekiq in the future. As the next step, we could introduce SidekiqEventReporter which would use composition over inheritance, and it will just log and increase additional metrics specific to Sidekiq.

        def configure_for_sidekiq
            ->(config) do
              config.handler = Gitlab::Memory::Watchdog::TermProcessHandler.new
              config.write_heap_dumps = write_heap_dumps?
              config.sleep_time_seconds = sidekiq_sleep_time
              config.monitors(&configure_monitors_for_sidekiq)
              config.event_reporter = EventReporter.new(Sidekiq.logger)
            end
          end

Events

          event_reporter.started

          event_reporter.stopped

          event_reporter.threshold_violated(monitor_name: monitor_name)

          event_reporter.strikes_exceeded(
            monitor_name: monitor_name,
            monitor_payload: monitor_payload
          )

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to: #383853 (closed)