Canvas usage-data query is expensive when workpads have images

[peterschretlen]

Kibana version: 6.7

Elasticsearch version: 6.7

Server OS version: macOS

Original install method: source

Describe the bug:
Canvas usage-data query to elasticsearch returns assets. If the asset is large, say a 10MB image, the query is slow and response is huge.

Steps to reproduce:

1. Turn on monitoring
2. Turn on usage data
3. Create new workpad
4. Insert an image element, upload an image

Expected behavior:
usage-data queries should not return large objects like assets.

Screenshots (if relevant):
Image being returned by query:

Any additional context:
Could we exclude assets from the source returned? This seems to address the issue

GET .kibana/_search?filter_path=hits.hits._source.canvas-workpad,-hits.hits._source.canvas-workpad.assets
{
  "query": {
    "bool": {
      "filter": {
        "term": {
          "type": "canvas-workpad"
        }
      }
    }
  }
}

[elasticmachine]

Pinging @elastic/kibana-canvas

[epixa]

We can exclude fields with the saved object client, so unless I’m missing something this should be a one line fix.

[shaunmcgough]

@clintandrewhall