Describe the feature:
Alert provides a way to define a default message that is going to be used when a user selects one of the available actions. This is possible because of a property called defaultActionMessage which is set while registering an alert.

I would like to propose a new property that could be called getDefaultActionMessage, which is a function and it would receive as an argument the alert parameters and would also return a string. But the difference is that I would be able to create custom messages based on the values the user selected in the alert conditions.

alertTypeRegistry.register({
    id: 'alert_id',
    name: 'amazing alert',
    iconClass: 'bell',
    alertParamsExpression: lazy(() => import('./XXX')),
    validate: () => ({errors: [],}),
    requiresAppContext: true,
    getDefaultActionMessage: (alertParams: AlertTypeParams) => {
       if(alertParams.aggregationType === 'avg'){
          return 'a nice message when user selects avg '
        } else if (alertParams.aggregationType === '95th') {
          return 'another custom message when user selects 95th'
       }
        return 'otherwise default message '
    }

Describe a specific use case for the feature:

On APM we'd like to set different messages based on user selection.

The new function, getDefaultActionMessage would be called every time a user adds a new action. In the image above depending on what option the user selected in the When field, we'd define a custom message for that option.