metrics: use labels for enums

[ripatel-fd]

Instead of

# HELP quic_stream_closed_end Number of streams closed. (gracefully closed)
# TYPE quic_stream_closed_end counter
quic_stream_closed_end{kind="quic",kind_id="0"} 294964918

# HELP quic_stream_closed_peer_reset Number of streams closed. (RESET_STREAM event received)
# TYPE quic_stream_closed_peer_reset counter
quic_stream_closed_peer_reset{kind="quic",kind_id="0"} 0

# HELP quic_stream_closed_peer_stop Number of streams closed. (STOP_SENDING events received)
# TYPE quic_stream_closed_peer_stop counter
quic_stream_closed_peer_stop{kind="quic",kind_id="0"} 0

# HELP quic_stream_closed_drop Number of streams closed. (dropped due to excessive concurrency)
# TYPE quic_stream_closed_drop counter
quic_stream_closed_drop{kind="quic",kind_id="0"} 148732482

# HELP quic_stream_closed_conn_abort Number of streams closed. (connection abort)
# TYPE quic_stream_closed_conn_abort counter
quic_stream_closed_conn_abort{kind="quic",kind_id="0"} 25101618

Generate

# HELP quic_stream_closed_end Number of streams closed. 
# TYPE quic_stream_closed_end counter
quic_stream_closed{kind="quic",kind_id="0",quic_stream_close="end"} 294964918
quic_stream_closed{kind="quic",kind_id="0",quic_stream_close="peer_reset"} 0
quic_stream_closed{kind="quic",kind_id="0",quic_stream_close="peer_stop"} 0
quic_stream_closed{kind="quic",kind_id="0",quic_stream_close="drop"} 148732482
quic_stream_closed{kind="quic",kind_id="0",quic_stream_close="conn_abort"} 25101618