Log canceled client requests at info level (#4947)

"zed serve" logs canceled client requests (specifically, those that end
due to a context.Canceled error appearing to originate from the request
context) at warn level, which includes a stack trace, with an alarming
message.

    {
      "level":"warn",
      "ts":1702997509.358625,
      "logger":"core",
      "msg":"Error",
      "request_id":"2ZlXC74Oeg2zEXQXX8ol8Ma1Lyd",
      "status":500,
      "error":"context canceled",
      "stacktrace":"..."
    }

Change the level to info, thereby omitting the stack trace, and change
the message to one less alarming.

    {
      "level":"info",
      "ts":1702998051.5594501,
      "logger":"core",
      "msg":"Request context canceled",
      "request_id":"2ZlYICXG8lrypWxyIxUBoM84qjT"
    }

service/request.go

@@ -1,6 +1,7 @@
 package service
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"io/fs"
@@ -43,6 +44,7 @@ func newRequest(w http.ResponseWriter, r *http.Request, c *Core) (*ResponseWrite
 		ResponseWriter: w,
 		Logger:         req.Logger,
 		marshaler:      m,
+		request:        req,
 	}
 	ss := strings.Split(r.Header.Get("Accept"), ",")
 	if len(ss) == 0 {
@@ -215,6 +217,7 @@ type ResponseWriter struct {
 	Logger    *zap.Logger
 	zw        zio.WriteCloser
 	marshaler *zson.MarshalZNGContext
+	request   *Request
 	written   int32
 }
 
@@ -256,6 +259,10 @@ func (w *ResponseWriter) Respond(status int, body interface{}) bool {
 }
 
 func (w *ResponseWriter) Error(err error) {
+	if err == context.Canceled && err == w.request.Context().Err() {
+		w.Logger.Info("Request context canceled")
+		return
+	}
 	status, res := errorResponse(err)
 	if status >= 500 {
 		w.Logger.Warn("Error", zap.Int("status", status), zap.Error(err))