Fix mdelem tracing

src/core/ext/transport/chttp2/transport/chttp2_transport.cc

@@ -1332,12 +1332,17 @@ static void log_metadata(const grpc_metadata_batch* md_batch, uint32_t id,
                          bool is_client, bool is_initial) {
   for (grpc_linked_mdelem* md = md_batch->list.head; md != nullptr;
        md = md->next) {
-    char* key = grpc_slice_to_c_string(GRPC_MDKEY(md->md));
+    if (is_valid_mdelem_index(md->md_index)) {
-    char* value = grpc_slice_to_c_string(GRPC_MDVALUE(md->md));
+      gpr_log(GPR_INFO, "HTTP:%d:%s:%s: hpack table index: %d", id, is_initial ? "HDR" : "TRL",
-    gpr_log(GPR_INFO, "HTTP:%d:%s:%s: %s: %s", id, is_initial ? "HDR" : "TRL",
+            is_client ? "CLI" : "SVR", md->md_index);
-            is_client ? "CLI" : "SVR", key, value);
+    } else {
-    gpr_free(key);
+      char* key = grpc_slice_to_c_string(GRPC_MDKEY(md->md));
-    gpr_free(value);
+      char* value = grpc_slice_to_c_string(GRPC_MDVALUE(md->md));
+      gpr_log(GPR_INFO, "HTTP:%d:%s:%s: %s: %s", id, is_initial ? "HDR" : "TRL",
+              is_client ? "CLI" : "SVR", key, value);
+      gpr_free(key);
+      gpr_free(value);
+    }
   }
 }