Add option to set event.created in integration/agent

[Oddly]

I would like to see a possibility to add an option in the Elastic Agent to add a field event.created with the timestamp of ingestion of the event. That is, a timestamp before the other processors will be run.
That way we can see how long an event took to travel from the source to the agent (in case of a TCP input) and, when we add a field with a timestamp as last processor, we can actually see how long (all) the processors of a agent took to run.

I would be even nicer to be able to set it (optionally) in the input options of an integration.

A diagram of the idea:

flowchart LR
    DataSource[Data Source] --> ElasticAgent
    subgraph ElasticAgent [Elastic Agent]
        SetEventCreated[Set event.created] --> BuiltInProcessors[Built-in Processors]
        BuiltInProcessors --> UserProcessors[User Processors]
        UserProcessors --> SetEventAgentOut[Set event.agent_out]
    end
    ElasticAgent --> Output[Output]

    classDef plain fill:#fff,stroke:#333,stroke-width:2px, border-radius: 5px;
    classDef fancy fill:#f9f7f7,stroke:#333,stroke-width:2px, border-radius: 5px;
    classDef agentBox fill:#83a4d4,fill-opacity:0.7,stroke:#333,stroke-width:2px, border-radius: 5px,stroke-dasharray: 5 5;
    classDef operation fill:#f7d1ba,stroke:#333,stroke-width:2px, border-radius: 5px;

    class DataSource,Output plain;
    class ElasticAgent agentBox;
    class SetEventCreated,BuiltInProcessors,UserProcessors,SetEventAgentOut operation;

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Edit: Clarified