telemetry: add gnmi tunnel proxy tool

[packethog]

Summary of Changes

This adds an example of a gRPC tunnel proxy for DZDs which enables the ability to subscribe to gNMI streaming telemetry samples without the requirement of dialing into devices. The tunnel proxy initiates a gRPC tunnel towards a collector, which then can be used to accept telemetry subscriptions over said tunnel. Subscription requests are then forwarded to the locally running gNMI server via a unix domain socket.

Testing Verification

Start collector instance with configuration to ask for the input pkt counter of Ethernet1 every 10s:

➜  cat config.yaml
insecure: true
username: admin
password: ""

subscriptions:
  sub1:
    stream-mode: SAMPLE
    paths:
      - /interfaces/interface[name=Ethernet1]/state/counters/in-pkts
    sample-interval: 10s

tunnel-server:
  address: :10000
  targets:
    - id: .*
      type: GNMI_GNOI
      config:
        subscriptions:
          - sub1
          - 
➜  gnmic --config ./config.yaml --use-tunnel-server subscribe
...

Start tunnel proxy on device which dials out to collector:

[root@e76554a34f51 app]# ./gnmi_tunnel -tunnel-server-addr 10.0.0.176:10000
time=2025-12-23T01:53:17.761Z level=INFO msg="tunnel client connected to" address=10.0.0.176:10000
time=2025-12-23T01:53:17.770Z level=INFO msg="tunnel client registered to" address=10.0.0.176:10000
time=2025-12-23T01:53:17.774Z level=INFO msg="new session established for target" target="{ID:dzd01 Type:GNMI_GNOI}"

Subscription responses begin to magically show up on the collector:

➜  gnmic --config ./config.yaml --use-tunnel-server subscribe
{
  "source": "dzd01",
  "subscription-name": "sub1",
  "timestamp": 1766454796083385625,
  "time": "2025-12-22T20:53:16.083385625-05:00",
  "updates": [
    {
      "Path": "interfaces/interface[name=Ethernet1]/state/counters/in-pkts",
      "values": {
        "interfaces/interface/state/counters/in-pkts": 9337
      }
    }
  ]
}
{
  "sync-response": true
}