DO NOT MERGE IP connectivity metrics: add transport field.

This patch adds a transport field dimension to IpConnectivityEvent for
replacing interface name, and eventually network ids.

At the moment the transport field is not populated.

Test: manually taking a diff against reference ipconnectivity.proto
Bug: 32198976

(cherry picked from commit 807124a7)

Change-Id: Idf2167973c322153659b8e755d8be973e70d972a