We are seeing lots of log spam related to cancelation of parse requests.
We haven't fully tracked this down yet, but the behaviour of spamming
logs in this case hides the real issue.

This adjusts the implementation in two places:
- We introduce a parseCanceled metric so we can still track this. This
  is used instead of parseFailed in case of cancelation. In this case we
  also do not emit the "Closing failed parser" even though we are. This
  is due to log spam.
- We introduce an ErrorFilter just for handleParseRequest that will
  elide Canceled from the logs. This is called per file. Note: we keep
  the error logging for canceled in the high level Parse handler which
  runs across all files.

Test Plan: go test. Unfortunetly that is as good as it gets for now, as
I investigate the root cause further I will test this code path (and
only merge once I have manually triggered and tested this code path).

(cherry picked from commit ae59c69)