Allow disabling of snowplow emitter buffering in staging

What does this MR do and why?

Allow disabling of snowplow emitter buffering in staging

• we find that there isn't enough volume in staging at times to reliably see events immediately, or even next day if the rails instance that has the buffer isn't visited enough.
• for experiment validation we like to verify in staging these things reliably, so we'll add a feature flag to be off except when we are looking to validate tracking reliably in staging.
• see https://gitlab.com/gitlab-org/gitlab/-/work_items/561744

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Related to #561744