Add lazy loading to APM AlertTypes

[gmmorris]

Hey team,
In the Alerting team we've been addressing an issue where Kibana is forced to pre-load a bunch of React components at startup that aren't actually needed until the Alerting UI or flyout are opened: #65678 (Once this PR is merged)

Sadly, this requires changes in how the AlertTypes are actually registered by solutions and so we'd ask that, when you have capacity to do so, you make a small change in how you register your alerts.

In the case of APM looks like we have two AlertTypes that need to change:

kibana/x-pack/plugins/apm/public/plugin.ts

Line 115 in 592e234

alertParamsExpression: ErrorRateAlertTrigger,

kibana/x-pack/plugins/apm/public/plugin.ts

Line 127 in 592e234

alertParamsExpression: TransactionDurationAlertTrigger,

Luckily, changing your AlertType to load lazily is relatively straight forward and you can look at the built-in IndexThreshold AlertType as an example:

1. Export your Expression component as a default export - as this is a hard requirement for lazy loading.
   

   kibana/x-pack/plugins/triggers_actions_ui/public/application/components/builtin_alert_types/threshold/expression.tsx

   Line 469 in 0736325

   export { IndexThresholdAlertTypeExpression as default };

2. Pass an React.LazyExoticComponent to the AlertType instead of the regular React.FC
   

   kibana/x-pack/plugins/triggers_actions_ui/public/application/components/builtin_alert_types/threshold/index.ts

   Line 18 in 0736325

   alertParamsExpression: lazy(() => import('./expression')),

That's about it.
Thanks 😊

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[smith]

@gmmorris added in #68806. Thanks for the detailed instructions!