feat(cloud_firestore, ios): DocumentSnapshotStreamHandler was not correctly listening on metadata changes

[PJakubec]

Description

On an iOS device, using includeMetadataChanges does not work stream correct metadata events for single document snapshots.
These events are not handled correctly:

• First query execution (works correctly on first app launch [because data is not yet in cache, but any subsequent launch will be handled incorrectly])
• Regaining/Losing connection to the database

Related Issues

Fixes: #12761

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Breaking Change

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