[Alerting] Index task manager health data into Kibana SM monitoring indices

[chrisronline]

Relates to #95197

As Kibana is responsible for more and more in the Stack, we need to ensure users have the necessary insights into the performance of Kibana from the Stack Monitoring UI.

Alerting currently has a health api for the task manager (api/task_manager/_health) but there are two main problems with it:

1. There is no corresponding UI that consumes the data and presents it in a useful way to users
2. The API is unusable if the Kibana instance is under too much load or is unresponsive (which is usually when you want to use the API!)

We should look to moving this data to also exist within the Kibana SM indices so we can eventually visualize it (and alert on it) within Stack Monitoring.

During 7.x, we'll need to make changes to two locations:

1. Internal collection -> https://github.com/elastic/kibana/blob/master/x-pack/plugins/monitoring/server/kibana_monitoring/bulk_uploader.ts
2. MB collection -> https://github.com/elastic/kibana/blob/ca17d931deaff7217dc7f75ae1ad6ff3be680543/src/plugins/usage_collection/server/routes/stats/stats.ts

We'll also need to update the mappings (found here) so we can perform aggregations across various fields.

It's not advised to just stick as much data as possible into the index, rather we need to be deliberate with which fields we want to index and map.

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[elasticmachine]

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

[mikecote]

Adding to backlog but up for consideration for 7.14 planning.