add debug logging

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 79d372c..6f548a3 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -433,6 +433,9 @@
 
     err = ngx_socket_errno;
 
+    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, ev->log, err,
+                   "http upstream recv(): %d", n);
+
     /*
      * we do not need to disable the write event because
      * that event has NGX_USE_CLEAR_EVENT type