Enabling appending time measurements to events can cause errors

[ppcad]

Enabling LOGPREP_APPEND_MEASUREMENT_TO_EVENT can cause FieldExists warnings and additionally raise FieldExists as an exception with stack-trace.

This problem started with pull request #696. It may be related to _add_fields_to in metrics.py.

Steps to reproduce

1. Run Logprep with the environment variable LOGPREP_APPEND_MEASUREMENT_TO_EVENT set to a truthy value
2. Run Logprep with any processor
3. Send an event containing {"processing_times": "foo"} to Logprep and ensure that a processor matches this event