[data.search.aggs] Sibling agg using Terms isn't able to sort the Terms agg by the metric

[wylieconlon]

Steps to reproduce:

1. Install the ecommerce sample data
2. Open an aggregated chart using the Kibana sample data
3. Begin configuring a metric
4. The metric is Overall Max
5. The metric is split by Terms of product.id
6. The sort order of the Terms agg is Avg of product.price
7. Set the Metric for Overall Max to also be Avg of product.price

Error:

Cannot retrieve search results

Validation Failed: 1: No aggregation [1-metric] found for path [1-bucket>1-metric];

Screenshot:

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)