[Logs UI] Register function(s) for homepage data retrieval

[jasonrhodes]

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 the log rate data specified by the Logs Return type
  • stats should include the values and names of each event.dataset value in the returned log data
  • series should include the histogram data for log rate within the given time range (partioned by event.dataset), in the format expected by Elastic Charts for a stacked bar chart / histogram

Notes:

• What will the chart show if there are more than 4 event.dataset values? Will the visualization scale? Will it use "top 4"?
• We don't currently have a generic log rate query so we will need to create this, not based on ML, so that it's available to all users

[elasticmachine]

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

[cauemarcondes]

@jasonrhodes here is an example of how you can register your function.

LogsFetchDataResponse: https://github.com/cauemarcondes/kibana/blob/obs-api/x-pack/plugins/observability/public/typings/data_handler/index.d.ts#L36

plugins.observability.dashboard.register({
  appName: 'infra_logs',
  fetchData: ({ startTime, endTime, bucketSize }) => {
    // TODO: fetch data.
    const logsData: LogsFetchDataResponse = {
      title: 'Logs',
      appLink: 'link to logs',
      stats: {
        nginx: {
          value: 100000,
        },
      },
      series: {
        nginx: {
          label: 'Nginx',
          coordinates: [{ x: 1, y: 2 }],
        },
      },
    };
    return Promise.resolve(logsData);
  },
  hasData: () => {
    // TODO: fetch has data.
    return Promise.resolve(true);
  },
});