The target server failed to respond and then the same request succeeds seconds later

## Expected Behavior

As a Graylog user, I expect all connections and requests to succeed if ES is not being overloaded.
## Current Behavior

Currently we are seeing in the Gl logs the error: ERROR [IndexFieldTypePollerAdapterES6] Couldn't get mapping for index : The target server failed to respond and then a few seconds later, the same request succeeds. We do not see anything in the ES logs related to that error message even with trace mode set.
## Possible Solution

## Steps to Reproduce (for bugs)

1. See HS-411015792 for details
2. This happened on the ugprade from 3.x to 4.x