ConnectivityMetricsEvent: add ifname, netid, transports

This patch adds new fields to ConnectivityMetricsEvent to make it more
symmetric to IpConnectivityEvent in ipconnectivity.proto.

Follow-up patches will start populating these fields for users of
IpConnectivityLog.

Test: unit tests updated, $ runtest frameworks-net passes
Bug: 34901696
Change-Id: I396767cdfcf38cce893c0d6e1f4524f12e3fdc64