Add metric for experiment hoisting errors

Ottomatarequested to merge
T398922_hoisting_metric into master
  • HostingError now subclasses ContextualError instead of ValidationError
  • HostingError raising code refactored to not try and look like a schema ValidationError
  • error_type is now a metric label on eventgate_validation_errors_total metric
  • Move error related code into ./lib/error.js -- this allowed me to use kafkaless eventgate-wikimedia-dev.js to test error event creation and error metrics.
  • Prep for T396359 by making it obvious on how to set which error types are emitted as error events
  • Also emit ExP MalformedHeaderError to the error stream - T396359

Bug: T398922 Bug: T396359

Edited by Ottomata

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