[Metrics Alerts] Fix alerting on a rate aggregation

[Zacqary]

Part of #56432

#57606 doesn't support the rate aggregator correctly. Use the networkTraffic function from x-pack/plugins/infra/common/inventory_models/shared/metrics/snapshot/network_traffic.ts to implement something like:

if (metric.aggregation === 'rate') {
    return networkTraffic(metric.type, metric.field);
  }
  return {
    custom: {
      [metric.aggregation]: {
        field: metric.field,
      },
    },
  };

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[Zacqary]

@simianhacker Can you remind me what the metric.type param is supposed to be in the example implementation? e.g. for avg of system.load.1 I know that I need to do:

avg: {
  field: 'system.load.1'
}

but I'm not sure what to do for rate of system.load.1

[Zacqary]

Never mind, realized it can just take an arbitrary string.