Add a 3rd flag count for quic_enable_http3_metadata_decoding.

Protected by FLAGS_quic_reloadable_flag_quic_enable_http3_metadata_decoding.

PiperOrigin-RevId: 617183882

quiche/quic/core/http/http_decoder.cc

diff --git a/quiche/quic/core/http/http_decoder.cc b/quiche/quic/core/http/http_decoder.cc
index 7a0828c..4ddcbcb 100644
--- a/quiche/quic/core/http/http_decoder.cc
+++ b/quiche/quic/core/http/http_decoder.cc
@@ -459,6 +459,7 @@
       if (enable_metadata_decoding_ &&
           current_frame_type_ ==
               static_cast<uint64_t>(HttpFrameType::METADATA)) {
+        QUIC_RELOADABLE_FLAG_COUNT_N(quic_enable_http3_metadata_decoding, 3, 3);
         continue_processing = visitor_->OnMetadataFrameEnd();
         break;
       }