Large field caps requests can cause memory pressure in smaller heaps

[benwtrent]

Description

When using a smaller heap, and gathering a large number of field caps, the memory pressure can be pretty large.

For example, some of the smaller heap sizes seen are < 1GB in size, but its possible for a field-caps response to use more than 200MB. One API request taking >20% of heap can cause issues.

This issue is to track two possible things:

• Should we and can we track field caps in the circuit breaker?
• Can we reject exceptionally large field-caps requests (by default) on tiny heaps?
• Is there any way to make heap usage for field-caps any cheaper?

These are rather, ambitious and potentially ambiguous questions. Opening this issue for potential discussion.

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)