HTTP/2: flow control debugging.

src/http/v2/ngx_http_v2_filter_module.c

diff --git a/src/http/v2/ngx_http_v2_filter_module.c b/src/http/v2/ngx_http_v2_filter_module.c
index 4ab7791..878020e 100644
--- a/src/http/v2/ngx_http_v2_filter_module.c
+++ b/src/http/v2/ngx_http_v2_filter_module.c
@@ -1079,6 +1079,10 @@
 ngx_http_v2_flow_control(ngx_http_v2_connection_t *h2c,
     ngx_http_v2_stream_t *stream)
 {
+    ngx_log_debug3(NGX_LOG_DEBUG_HTTP, h2c->connection->log, 0,
+                   "http2:%ui available windows: conn:%uz stream:%z",
+                   stream->node->id, h2c->send_window, stream->send_window);
+
     if (stream->send_window <= 0) {
         stream->exhausted = 1;
         return NGX_DECLINED;