Stub Snowplow before all specs

Based on the discussion at #345488 (closed), one of the ideas worth pursuing is to enable the :snowplow RSpec metadata globally for all specs. The logic of which it's currently at spec/support/stub_snowplow.rb.

File at eae5f187

# frozen_string_literal: true

module StubSnowplow
  def stub_snowplow
    # Using a high buffer size to not cause early flushes
    buffer_size = 100
    # WebMock is set up to allow requests to `localhost`
    host = 'localhost'

    # rubocop:disable RSpec/AnyInstanceOf
    allow_any_instance_of(Gitlab::Tracking::Destinations::Snowplow)
      .to receive(:emitter)
      .and_return(SnowplowTracker::Emitter.new(host, buffer_size: buffer_size))
    # rubocop:enable RSpec/AnyInstanceOf

    stub_application_setting(snowplow_enabled: true)

    allow(SnowplowTracker::SelfDescribingJson).to receive(:new).and_call_original
    allow(Gitlab::Tracking).to receive(:event).and_call_original # rubocop:disable RSpec/ExpectGitlabTracking
  end
end

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The related POC MR laid out the actual challenges we'd like to overcome with this issue (quoting from #345488 (comment 743214475)):

We need to figure out a better way to spy on Gitlab::Tracking.event. As of now, it just expects that the function was called. Not if the function was executed properly. Maybe by throwing instead of an early return from the Destination 👀 (thanks to @pskorupa for the suggestion)

We would need a way to opt-out/disable Snowplow. This can be done with stub_application_setting(snowplow_enabled: false). This method doesn't work at the moment because of the current spy

Testing

All specs related to the module StubSnowplow should be passing and make sure QA pipelines run smoothly.