Rename flag --quic_export_server_num_packets_per_write_histogram to --quic_export_write_path_stats_at_server.

I plan to add a couple of histograms under the same flag.

PiperOrigin-RevId: 339311828
Change-Id: Icf2f912914d05841e08637c1c697391b89bec08a
diff --git a/quic/core/quic_packet_creator.cc b/quic/core/quic_packet_creator.cc
index 662754e..3db37ed 100644
--- a/quic/core/quic_packet_creator.cc
+++ b/quic/core/quic_packet_creator.cc
@@ -1464,7 +1464,7 @@
   FlushCurrentPacket();
   SendRemainingPendingPadding();
   flusher_attached_ = false;
-  if (GetQuicFlag(FLAGS_quic_export_server_num_packets_per_write_histogram)) {
+  if (GetQuicFlag(FLAGS_quic_export_write_path_stats_at_server)) {
     if (!write_start_packet_number_.IsInitialized()) {
       QUIC_BUG << "write_start_packet_number is not initialized";
       return;