v0.2.1

Connection metrics. Servers can now see how many WebSocket, SSE and grpcws
connections are open, why each one closed (including clients dropped for
not reading), and how many frames went each way, without hand-rolling it.
The hooks have no metrics dependency, and servicekit/prom exports them to
Prometheus (issues 11 and 13, PR 58).

Added

• http.ConnObserver, with ConnInfo and CloseReason. Set it on
  BaseConn.Observer / BaseSSEConn.Observer, once on
  WSConnConfig.Observer / SSEConnConfig.Observer, or on
  grpcws.ConnOptions.Observer. It hears every open, every close with a
  reason (client_close, server_close, write_timeout, write_error,
  pong_timeout, read_error, start_failed) and lifetime, and every frame
  sent or received. A nil observer costs nothing.
• servicekit/prom: ConnMetrics implements ConnObserver for Prometheus,
  and HTTPMetrics is request middleware labelled by mux route template. The
  root go.mod now requires prometheus/client_golang v1.24.1, but
  http/ and middleware/ don't import it, and make check-deps now fails if
  they do.
• Load tests for WebSocket (100 connections, 500 connect/disconnect cycles
  with a goroutine-leak check, broadcast, a slow consumer) and SSE (100
  connections, hub churn). They skip under -short.
• make exercise: cmd/grpcws-demo -exercise opens 300 grpcws
  subscribers, 10 of which never read, and checks /metrics against what the
  driver did. CI runs it. The demo serves /metrics and takes
  -write-timeout and -close-on-stream-end.

Fixed

• cmd/grpcws-demo: a room sent its join/leave/action events while holding
  its lock, and its broadcast loop needs that lock to drain them, so many
  players leaving at once could deadlock it.