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.

[karimra]

Should be added in v0.39.0, please give it a try.

[XioNoX]

I'm not the initial requester but we've been waiting for it as well :)
Tested and works well for us. Thank you for implementing it !
Quick dashboard.

[hellt]

oh, wow, Wikimedia uses gnmic then, @XioNoX ?

[XioNoX]

Yep, we're only at the beginning, but it has been working very well for us so far. Thanks for the tool!
You can find more doc on https://wikitech.wikimedia.org/wiki/Network_telemetry

[aned]

Nice, thank you!