[Task Manager] Using track_total_hits in workload agg

[ymao1]

Resolves #98624

Summary

1. Added track_total_hits: true to the workload aggregation query in order to get an accurate count of tasks.
2. Added a size parameter to the task type term aggregation. Default size is 10 so the aggregation would never return more than 10 task types

To verify this, I created an Always Firing rule running every 15 seconds and attached some actions to it. Then I let it run for a while to build up the number of tasks. Eventually, doing a curl -X GET api/task_manager/_health should return a response like this:

{
    "id": "5b2de169-2785-441b-ae8c-186a1936b17d",
    "timestamp": "2021-05-05T17:09:00.930Z",
    "status": "OK",
    "last_update": "2021-05-05T17:08:59.839Z",
    "stats": {
        "configuration": { ... },
        "runtime": { ... },
        "workload": {
            "count": 12504,                        <---------------- this count should exceed 10,000
            "task_types": {
                  "actions:.pagerduty": {.    <----------------- there should be more than 10 task types listed
                      "count": 12480,
                      "status": {
                          "failed": 12480,
                      }
                  },
                 other task types here
            }  
        }
    }
}

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: f401fcd
• Storybooks Preview

Metrics [docs]

✅ unchanged

History

• 💚 Build #124462 succeeded d4e24ec

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @ymao1

[pmuellr]

LGTM

[chrisronline]

LGTM!

[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.13
✅	7.x

The backport PRs will be merged automatically after passing CI.