fix(core): Record byte discards for event processor drops (#5718)

* fix(core): Record byte discards for event processor drops

Track log and trace metric byte counts when event processors drop log or metric events. This keeps EventProcessor discard accounting aligned with before-send and queue-overflow paths.

Co-Authored-By: Claude <noreply@anthropic.com>

* changelog

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Co-authored-by: Claude <noreply@anthropic.com>