metrics: add metrics for pod and namespace count

[shashankram]

Description:
Adds metrics to report the number of monitored
namespaces and pods participating in the mesh.

Part of #902

Signed-off-by: Shashank Ram shashr2204@gmail.com

Affected area:

• New Functionality [ ]
• Documentation [ ]
• Install [ ]
• Control Plane [ ]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [X]
• SMI Policy [ ]
• Security [ ]
• Tests [ ]
• CI System [ ]
• Performance [ ]
• Other [ ]

Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?
  No

[ksubrmnn]

This looks good.

In the case of pods, how do we consider if we delete the mesh and the pods still have their sidecars. Then a new mesh is created and the namespace is added again. How do those old pods get counted here?

[shashankram]

This looks good.

In the case of pods, how do we consider if we delete the mesh and the pods still have their sidecars. Then a new mesh is created and the namespace is added again. How do those old pods get counted here?

The metrics are pertaining to a particular instance of osm-controller. If we delete the mesh (osm-controller), and a new mesh (osm-controller) is created, the controller will receive the same events as the previous controller when its informer caches are initialized. This means metrics for namespaces and pods that were previously part of the mesh, will show the same count before and after they are associated with a different controller. This can be tested by restarting the osm-controller pod as an example.