This commit adds two new fields, tx_bytes and rx_bytes, to the
UpstreamEvent message. These fields provide information about the
amount of data transferred during the event, which can be useful for
network performance analysis. The current tx/rx usages are always
zero, but it will be updated in a later change to reflect the actual
usage. No changes were made to the existing fields upstream_type and
duration_millis.
Test: atest TetheringMetricsTest
Change-Id: I9e1043b3fff5aa3f548b8696bac7e5bc4271c77c
b5ee365614