Flush metadata packet after write

The flush in discard buffer mode happens only at end of trace and the
packet is discarded if the buffer is already full. So flush after writing the
metadata packet.

Change-Id: I4cc75a03909629eace1e8bdfcb4eeb2239ad1309
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518520
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824030}

services/tracing/public/cpp/perfetto/trace_event_data_source.cc

@@ -360,15 +360,17 @@ void TraceEventMetadataSource::GenerateMetadata(
     generator.Run(chrome_metadata, privacy_filtering_enabled);
   }
 
-  if (privacy_filtering_enabled) {
-    return;
-  }
-
+  if (!privacy_filtering_enabled) {
     ChromeEventBundle* event_bundle = trace_packet->set_chrome_events();
-
     for (auto& generator : *json_generators) {
       GenerateJsonMetadataFromGenerator(generator, event_bundle);
     }
+  }
+
+  // Force flush the packet since the default flush happens at end of
+  // trace, and the packet can be discarded then.
+  trace_packet->Finalize();
+  trace_writer->Flush();
 }
 
 void TraceEventMetadataSource::StartTracing(