[FEATURE] Implement alerting stats API

[AWSHurneyt]

Is your feature request related to a problem? Currently, collecting stats relating to adoption, usage, performance, etc. of the alerting plugin requires calling various API (e.g., GetMonitors to check whether there are any enabled monitors, GetAlerts to see if any monitors are currently executing with errors, etc.).

What solution would you like? A single, generalizable stats API would be helpful for providing a quick solution to capturing metrics relating to the usage and performance of the alerting plugin. This API should support enhancement so we can easily add more metrics to output as more features are released. Some immediate examples of useful metrics to output include:

1. Number of enabled monitors by monitor_type
2. Number of each monitor_type with alerts that have the ERROR status
3. Tracking the execution history of monitors even when they do not generate alerts; last successful run
4. Metric to track consistent monitor failures
5. Number of each monitor_type configured using the UI as opposed to curl/devtools commands; look for monitors that have ui_metadata. The number of monitors configured using the extraction query editor vs. visual editor could indicate that the visual editor should be enhanced to accommodate more complex queries

[engechas]

Some other ideas for stats to expose are discussed in #836