Support enriching received data with additional global labels

In 8.0, when we replaced the libbeat Elasticsearch output, we dropped support for the `fields` configuration.

The main reason we dropped support for `fields` was so we could move away from the free-form bag-of-key-value pairs libbeat model, which required inefficient reflection-based encoding. At that time we expected users could add fields with an ingest pipeline.

As it turns out, some users want to run multiple APM Servers, and have data enriched according to which APM Server receives the data. For example, one may have an APM Server per organisational department (IT, engineering, etc.), and label data accordingly.

To enable this, we should either reinstate `fields`, or introduce a new mechanism for setting global labels like [Jaeger agent tags](https://www.jaegertracing.io/docs/1.46/deployment/#agent-level-tags).