[2.0] Add example for dropping attributes

[strawgate]

Most attributes are useless and they consume a lot of space.

We should publish an example ingest pipeline where we drop attributes, like --

PUT _ingest/pipeline/metrics-homeassistant-pipeline
{
  "metrics-homeassistant-pipeline": {
    "description": "Pipeline for HomeAssistant dataset",
    "processors": [
      {
        "dot_expander": {
          "field": "hass.entity.attributes"
        }
      },
      {
        "remove": {
          "ignore_missing": true,
          "field": "hass.entity.attributes",
          "if": "ctx.hass.entity.domain != \"sensor\""
        }
      }
    ]
  }
}