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

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

related to: #379196 (closed)