Attach Outputs to a subscription

[mwdomino]

This PR is a followup to the discussion in #198

It includes the ability to specify outputs under a subscription in configuration. If specified, all data from the subscription will be sent to the given output(s). If there are no outputs specified under a subscription then it will exhibit the current behavior and flood the messages out to each defined output.

I've tested this locally using containerized Arista (cEOS) switches with the following config file and all works as I expect:

username: admin
password: admin
insecure: true
subscribe-backoff: 1s

targets:
  localhost:6030:
    subscriptions:
      - arista_image
      - arista_arp
      - arista_ethernet_state
  localhost:6031:
    subscriptions:
      - arista_image
      - arista_arp
      - arista_ethernet_state

subscriptions:
  arista_ethernet_state:
    paths:
      - "/interfaces/interface/ethernet/state"
    mode: stream
    stream-mode: ON_CHANGE

  arista_image:
    paths:
      - "eos_native:/Eos/image"
    mode: stream
    stream-mode: ON_CHANGE
    outputs: 
      - prom1
      - prom2

  arista_arp:
    paths:
      - "eos_native:/Smash/arp/status/arpEntry"
    mode: stream
    stream-mode: ON_CHANGE
    outputs: 
      - prom3

outputs:
  prom1:
    type: prometheus
    listen: :9801
    path: /metrics
    metric-prefix: "prom1"
  prom2:
    type: prometheus
    listen: :9802
    path: /metrics
    metric-prefix: "prom2"
  prom3:
    type: prometheus
    listen: :9803
    path: /metrics
    metric-prefix: "prom3"

For this test I just used 3 prometheus outputs so that I could curl localhost:<port>/metrics to verify only expected messages were received.

In this example:

• arista_arp - will only be sent to prom3
• arista_image - will be sent to prom1 and prom2
• arista_ethernet_state - will be sent to prom1, prom2, and prom3 as no output is specified (current behavior)

As this is my first time contributing to the project please let me know if there's anything you'd like me to change or if there is a different process to follow when opening up PRs like this!

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[mwdomino]

I believe the re-scan after signing the CLA was successful.

[karimra]

Thanks for the contribution!