Function addEventModifier

addEventModifier(
    eventModifier: (
        jsonEvent: Readonly<JSONEvent>,
        eventContext?: EventContext,
    ) => JSONEvent,
): () => void
Safe wrapper for dynatrace.addEventModifier which reverts to a noop if RUM JavaScript is not available.
Parameters
- eventModifier: (jsonEvent: Readonly<JSONEvent>, eventContext?: EventContext) => JSONEvent
  The modifier function to modify a given (readonly) JSONEvent. It receives an optional context depending on the type of event, if available. The returned event must be a new object, otherwise modification will be ignored.
Returns () => void
The unsubscriber function to turn off event modification, or undefined if RUM JavaScript is not available.
Example
```
import { addEventModifier } from '@dynatrace/rum-javascript-sdk/api';

const unsubscribe = addEventModifier((event, context) => {
    return {
        ...event,
        'event_properties.user_tier': 'premium'
    };
});

// Later, remove the modifier
unsubscribe?.();
```