This adds a relatively simple new field to all events:

{ ... meta: "Freeform set of key:value pairs e.g. { 'ewma': 12345 }", ... }

I came across this problem when working on a relatively simple use-case:

Emit data when the metric is one (or more) standard deviations above
(or below) the EWMA of the same metric.

To make this decision you need to know the metric, the ewma and the stdDev. Before adding meta I was having the ewma stream simply attach tags:

  {
    // ...
    "tags": ["ewma:12345.67", "stdDev:234.5"],
    // ...
  }

This is all well and good, but it felt wrong to be serializing and deserializing these values later on in the stream processing pipeline. With meta I end up with:

  {
    // ...
    "meta": {
      "ewma": 12345.67, 
      "stdDev": 234.5
    }
    // ...
  }

and don't have to deserialize again.