This tracks and emits metrics for the following AWS SDK account ID features...

This tracks and emits metrics for the following AWS SDK account ID features (corresponding metric value in parenthesis):

ACCOUNT_ID_MODE_PREFERRED (P)
ACCOUNT_ID_MODE_DISABLED (Q)
ACCOUNT_ID_MODE_REQUIRED (R)
RESOLVED_ACCOUNT_ID (T)

Testing:
- Integration tests for tracking account ID endpoint modes
- Unit tests validate User-Agent metric emission