Function addEventModifier

addEventModifier(
    eventModifier: (
        jsonEvent: Readonly<JSONEvent>,
        eventContext?: EventContext,
    ) => JSONEvent,
    timeout?: number,
): Promise<() => void>
Async wrapper for dynatrace.addEventModifier.
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.
- timeout: number = DEFAULT_AGENT_TIMEOUT
  Timeout in milliseconds to wait for the RUM JavaScript to be available (default: 10000)
Returns Promise<() => void>
A promise that resolves to the unsubscriber function to turn off event modification.
Throws
A DynatraceError if the RUM JavaScript is not available within the timeout, or the namespace is overridden with an incompatible object
Example
```
import { addEventModifier } from '@dynatrace/rum-javascript-sdk/api/promises';

const unsubscribe = await addEventModifier((event) => ({
    ...event,
    'event_properties.environment': 'production'
}));
```