Excessive Metaspace usage when Error Prone enabled

As mentioned in https://github.com/google/error-prone/issues/2786#issuecomment-1003368760, we are planning to move away from forking `javac`. However, after disabling the `fork` option, we noticed that the metaspace (off-heap) usage increased significantly (220 MiB -> 1.8 GiB). Please have a look at the graphs below.

I don't have a reproduction case at the moment, but we don't have anything custom, only some command-line options to enable/disable some checks. Maybe it's worth mentioning; we also use [NullAway](https://github.com/uber/NullAway); however, disabling it didn't change the metaspace usage.

Depending on the feedback, I am happy to help reproduce this in an open-source repository. Please let me know.

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I run `mvn -T 4 clean install -DskipTests` and attached `jconsole`.

#### Error Prone disabled

#### Error Prone enabled