Incorrect negative total metrics when enabling / disabling AirPlay

[defagos]

As an engineer I want correct reliable data that can be plotted with confidence. There is currently an issue affecting metrics when AirPlay is enabled or disabled, reproduced as follows:

• Play some content.
• Open the metrics debugging view.
• Connect to AirPlay and observer that data volume / media requests suddenly provide a negative contribution, likely equal to the current total.

Acceptance criteria

• The issue described above has been fixed.

Tasks

• [ ] Fix issue.
• [ ] File radar.