MINOR: quic: QUIC stats counters handling

MINOR: quic: QUIC stats counters handling

First commit to handle the QUIC stats counters. There is nothing special to say
except perhaps for ->conn_openings which is a gauge to count the number of
connection openings. It is incremented after having instantiated a quic_conn
struct, then decremented when the handshake was successful (handshake completed
state) or failed or when the connection timed out without reaching the handshake
completed state.