'Records ejected from the Engine Reporting signature cache' error

## Description

The following error shows up on the production server a lot. Reloading the server seems to make it stop but it always comes back after some time.

> This server is processing a high number of unique operations. A total of _n_ records have been ejected from the Engine Reporting signature cache in the past interval. If you see this warning frequently, please open an issue on the Apollo Server repository.

## Expected behavior:
No errors.

## Actual behavior:
A lot of errors.

## Versions
Apollo server: 2.9.11
Node: 12.13.0
Ubuntu: 16.04.6

## Runnable reproduction
Unfortunately I can't provide that since this error _only_ shows up on our production server. Understanding what causes this error and whether any action is required would be greatly appreciated.

The report names no source file, test, or entry point, and provides no runnable reproduction because the error occurs only in production. Start by tracing the Engine Reporting signature-cache warning in the Apollo Server codebase; done means identifying its cause and documenting whether the warning requires operator action.

Written by the indexing model from the issue text.