http: don't log an error if the request is cancelled

Now that we have at least one endpoint that uses context for cancellation we can
encounter this scenario where the returned error is a context.Cancelled or
context.DeadlineExceeded.

If the request.Context().Err() is not nil, then we know the request itself was cancelled, so
we can log a different message at Info level, instad of the error.

agent/http.go

@@ -432,12 +432,20 @@ func (s *HTTPHandlers) wrap(handler endpoint, methods []string) http.HandlerFunc
 		}
 
 		handleErr := func(err error) {
-			httpLogger.Error("Request error",
-				"method", req.Method,
-				"url", logURL,
-				"from", req.RemoteAddr,
-				"error", err,
-			)
+			if req.Context().Err() != nil {
+				httpLogger.Info("Request cancelled",
+					"method", req.Method,
+					"url", logURL,
+					"from", req.RemoteAddr,
+					"error", err)
+			} else {
+				httpLogger.Error("Request error",
+					"method", req.Method,
+					"url", logURL,
+					"from", req.RemoteAddr,
+					"error", err)
+			}
+
 			switch {
 			case isForbidden(err):
 				resp.WriteHeader(http.StatusForbidden)