Add telemetry error to monitor abrupt changes in user tab count

[data-sync-user]

We should be able to monitor dramatic changes in tab counts between when the app is backgrounded, and a short time after it is resumed. Apart from Close All Tabs actions, there should be no scenarios in which a dramatic drop in tab count can be observed. As a result this could be used as a means to help monitor bugs that cause potential tab loss for users.

Plan for now is to add a MetricKit error so that this can potentially be added to our health dashboard.

Ex: https://github.com/mozilla-mobile/firefox-ios/blob/986ee486346c652655ae4f5409168e4f4ce9abcb/firefox-ios/Client/Telemetry/MetricKit/MetricKitWrapper.swift#L47

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