Feature request: Improve observability in api-server metrics implementation

[aned]

This is a follow up on #498 , currently api-server /metrics implementation doesn't allow to properly monitor targets up/down.

api-server:
  address: :7890
  enable-metrics: true

Once the target becomes gnmi unreachable, the current metric gnmic_subscribe_number_of_received_subscribe_response_messages_total disappears.

There should be a deterministic way in /metrics to tell whether a target up or down.