To facilitate observability homepage data retrieval, each observability app must register a function or functions that will retrieve data in the correct format according to the registration contract provided by the observability app.

Design: #68176

AC:

• On plugin start, we register a function using plugins.observability.dashboard.register() that retrieves metrics data specified by the Metrics Return type
  • stats should include these values:
    • hosts: total over the time range
    • cpu: average over the time range
    • memory: average over the time range
    • disk usage: average over the time range
    • inbound traffic: total over the time range
    • outbound traffic: total over the time range
  • series should include [tbd, not sure yet]

Notes:

• Are these type of average values interesting/useful?
• Should we swap in a metrics explorer time series graph instead, showing these values over the whole selected time range?
• There may be problems getting "disk usage" and we may need to punt on that for now