bug: add payload validation filters are applied

The RUM agent let users to define filters for the data that is going to be sent to the APM server. This can be done using [addFilter API](https://www.elastic.co/docs/reference/apm/agents/rum-js/agent-api#apm-add-filter). The filter function can discard payloads but also can mutate them as shown in the example in the docs. There is a possibility that such filter modifies the payload in wrong way and as a consequence the agent sends incorrect payloads to the server. There is no problem on the server side since it discards the wrong request but we should avoid sending them.