Introduce multiple Monitor support for the Watchdog

What does this MR do and why?

This MR extracts Monitoring logic from Watchdog.

This allows us to introduce multiple monitors to run within the watchdog.

We can even enable experimental monitors directly in the initializer and have different ones for Puma and Sidekiq.

Configuration:

 watchdog = Gitlab::Memory::Watchdog.new
  watchdog.configure do |config|
    config.handler = Gitlab::Memory::Watchdog::NullHandler.instance
    config.sleep_time_seconds = 30

    # config.monitor.use MonitorClass, args**, &block
    config.monitor.use Gitlab::Memory::Watchdog::Monitors::HeapFragmentationMonitor
    config.monitor.use Gitlab::Memory::Watchdog::Monitors::MemoryGrowthMonitor
  end
  
Gitlab::BackgroundTask.new(watchdog).start

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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: #379196 (closed)

changed milestone to %15.4

added cloud connectoractive featureenhancement groupcloud connector typefeature labels

requested review from @mkaeppler

assigned to @nmilojevic1

added devopsdata stores sectioncore platform labels

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added backend label

	1 Warning
	This merge request is quite big (1263 lines changed), please consider splitting it into multiple merge requests.

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backend	Amparo Luna (@a_luna) (UTC-5, 7 hours behind @nmilojevic1)	Jan Provaznik (@jprovaznik) (UTC+2, same timezone as @nmilojevic1)

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Generated by Danger

added 1 commit

• 36e19e93 - Fix namespacing for monitors

Compare with previous version

mentioned in merge request !96241 (merged)

changed the description

I think this is a nice approach I would say let's park this for now since we don't know yet if or how this would be used outside of Puma, but come back to it when we look at Sidekiq, where it might be useful to have a more modular approach?

closed

changed the description

mentioned in merge request !95911 (closed)

mentioned in issue #370239 (closed)

mentioned in issue #368692 (closed)

mentioned in issue #368399 (closed)

mentioned in epic &8415 (closed)