Add caveat to noisy comment, push to dbg mode

src/core/ext/transport/chttp2/transport/writing.cc

@@ -139,22 +139,27 @@ static bool update_list(grpc_chttp2_transport* t, grpc_chttp2_stream* s,
 
 static void report_stall(grpc_chttp2_transport* t, grpc_chttp2_stream* s,
                          const char* staller) {
-  gpr_log(
-      GPR_DEBUG,
-      "%s:%p stream %d stalled by %s [fc:pending=%" PRIdPTR
-      ":pending-compressed=%" PRIdPTR ":flowed=%" PRId64
-      ":peer_initwin=%d:t_win=%" PRId64 ":s_win=%d:s_delta=%" PRId64 "]",
-      t->peer_string, t, s->id, staller, s->flow_controlled_buffer.length,
-      s->compressed_data_buffer.length, s->flow_controlled_bytes_flowed,
-      t->settings[GRPC_ACKED_SETTINGS]
-                 [GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE],
-      t->flow_control->remote_window(),
-      static_cast<uint32_t> GPR_MAX(
-          0,
-          s->flow_control->remote_window_delta() +
-              (int64_t)t->settings[GRPC_PEER_SETTINGS]
-                                  [GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE]),
-      s->flow_control->remote_window_delta());
+  if (grpc_flowctl_trace.enabled()) {
+    gpr_log(
+        GPR_DEBUG,
+        "%s:%p stream %d moved to stalled list by %s. This is FULLY expected "
+        "to happen in a healthy program that is not seeing flow control stalls."
+        " However, if you know that there are unwanted stalls, here is some "
+        "helpful data: [fc:pending=%" PRIdPTR ":pending-compressed=%" PRIdPTR
+        ":flowed=%" PRId64 ":peer_initwin=%d:t_win=%" PRId64
+        ":s_win=%d:s_delta=%" PRId64 "]",
+        t->peer_string, t, s->id, staller, s->flow_controlled_buffer.length,
+        s->compressed_data_buffer.length, s->flow_controlled_bytes_flowed,
+        t->settings[GRPC_ACKED_SETTINGS]
+                   [GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE],
+        t->flow_control->remote_window(),
+        static_cast<uint32_t> GPR_MAX(
+            0,
+            s->flow_control->remote_window_delta() +
+                (int64_t)t->settings[GRPC_PEER_SETTINGS]
+                                    [GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE]),
+        s->flow_control->remote_window_delta());
+  }
 }
 
 static bool stream_ref_if_not_destroyed(gpr_refcount* r) {