[Refactor] Make exporter service become juju Object base.

[jneo8]

Currently the logic to control the service is base on the BaseExporter class, which controls the life-cycle of the service. And we control this exporter class life-cycle in the main charm class.

The refactor idea is that we re-create the exporter service class inherit from operator framework's Object class

• This means every exporter service need to handle their install/config-change/remove hooks.
• With this change, every time we need to add a new exporter to this charm, we only need to create another exporter service class and register them to the main charm class. Don't have to change the skeleton of main charm class.
• The exporter service class should handle it's own resource download/attach. The situation right now is the attach resource logic belong to the main charm class. From my point of view, those hardware resource should always belong to one specific exporter, e.g., the storcli belong to hardware-exporter and smartctl exporter binary belong to smartctl-exporter.

sudo refactor code

import ops

class HardwareObserverCharm(ops.CharmBase):

    def __init__(self, args) -> None
      self.hardware_exporter = HardwareExporterService(self)
      self.smart_exporter = SmartExporterService(self)
      ...
      

class HardwareExporterService(ops.Object):
    
    strategies = [StrategyClassA(), StrategyClassB()]
    
    def __init__(self, charm):
        self._charm = charm
        self.framework.observer(self._charm.on.install, self._on_install)
        self.framework.observer(self._charm.on.config_changed, self._on_config_changed)
        
    def _on_install(self, event):
        """Install resources and exporter service."""
        for strategy in self.strategies:
            strategy.install()
        
        # Install exporter service
        ...
        
    def _on_config(self, event):
        """Configure."""
        # Render/Set configurations
        ...


class SmartExporterService(ops.Object):
    
    strategies = [StrategyClassC(), StrategyClassD()]
    
    ...