netty: Increase HPACK encoder bucket count to 64 (#13077)

Increase the HPACK encoder bucket count from 16 to 64 for both clients
and servers, matching Netty’s new default. On newer Netty versions, this
permits encoder tables up to 4 KiB within the peer’s advertised limit.
Advertised table sizes and the Netty dependency version are unchanged.

Use 64 rather than 128 following review: the adjusted benchmark showed
comparable performance, while larger dynamic tables need improvements to
Netty’s encoder.

Related to #12973. Discovered by @ejona86.

Co-authored-by: Xirp Agent (gpt-6.1-sol) <xirp-agent@users.noreply.ghe.spotify.net>

Author: petedmarsh

netty/src/main/java/io/grpc/netty/NettyClientHandler.java

@@ -176,7 +176,7 @@ static NettyClientHandler newHandler(
     Http2HeadersDecoder headersDecoder = new GrpcHttp2ClientHeadersDecoder(maxHeaderListSize);
     Http2FrameReader frameReader = new DefaultHttp2FrameReader(headersDecoder);
     Http2HeadersEncoder encoder = new DefaultHttp2HeadersEncoder(
-        sensitivityDetector(neverIndexedMetadataKeys), false, 16, Integer.MAX_VALUE);
+        sensitivityDetector(neverIndexedMetadataKeys), false, 64, Integer.MAX_VALUE);
     Http2FrameWriter frameWriter = new DefaultHttp2FrameWriter(encoder);
     Http2Connection connection = new DefaultHttp2Connection(false);
     UniformStreamByteDistributor dist = new UniformStreamByteDistributor(connection);

netty/src/main/java/io/grpc/netty/NettyServerHandler.java

@@ -189,7 +189,7 @@ static NettyServerHandler newHandler(
     Http2HeadersEncoder encoder = new DefaultHttp2HeadersEncoder(
         NettyClientHandler.sensitivityDetector(neverIndexedMetadataKeys),
         false,
-        16,
+        64,
         Integer.MAX_VALUE);
     Http2FrameWriter frameWriter =
         new Http2OutboundFrameLogger(new DefaultHttp2FrameWriter(encoder), frameLogger);