Avoid returning metrics for reserved items

[defagos]

Description

This PR avoids returning meaningless metrics when the player current item is a loading or failing item:

Before	After

Changes made

Self-explanatory. Note that metrics might still be returned for items with a valid non-reserved URL but failing to play.

Checklist

• [x] APIs have been properly documented (if relevant).
• [x] The documentation has been updated (if relevant).
• [x] New unit tests have been written (if relevant).
• [x] The demo has been updated (if relevant).