Update the Prometheus metrics page

[adglkh]

There are a few problems when I take a look the
https://documentation.ubuntu.com/anbox-cloud/en/latest/reference/prometheus/#access-endpoint

1. We are still referencing anbox cloud appliance(epoch=0) in this page in lots of places E.g.

   For the Anbox Cloud Appliance: juju run --wait=5m -m appliance:anbox-cloud --unit ams/0 -- unit-get private-address
   

2. The following metric is not correct

• anbox_stream_gateway_agents_total Number of active and unresponsive agents
  ->
• anbox_stream_gateway_agents_total Number of active agents
• anbox_stream_gateway_agents_unresponsive Number of unresponsive agents

I haven't reviewed all the other metrics yet, but it’s worth checking them for verification.