gfe-relnote: Refactor how QuicConnectionStats.slowstart_duration is calculated in QUIC BBR1, and start populate it in QUIC BBR2. Not protected. (Worst case we get some incorrect values in transport connection stats) Similar to slowstart_duration, I will add drain_duration, probe_rtt_duration, probe_bw_(down|up|cruise)_duration, in a follow up CL. PiperOrigin-RevId: 282224334 Change-Id: I873c39dcd47eb03d22d245cdfbf13aabc1a814e6