nozzle: collect metrics for errors from firehose disconnections
nozzle: collect metrics for errors from firehose disconnections

Helper operators track disconnect metrics:

- heartbeater.firehose.errors.close.normal_close: Server disconnected us
  gracefully. This generally means the nozzle is not responding fast
  enough to events and the nozzle should be horizontally scalled.
- heartbeater.errors.close.policy_violation: Web socket policy was
  violated. The network may have been temporarily unavalible causing the
  connection to close. The nozzle may also not be responding fast enough
  to events.

Based off of: https://github.com/cloudfoundry-community/splunk-firehose-nozzle/blob/4b8a943cf88fdd8daf7cf01a47e44ecb8ab467c6/nozzle/nozzle.go#L78

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