6efa9ee3 added `volatile` to `attributes` after TSAN detected a data
race that was added in 91d15ce4. The race was because attributes may be
read from another thread after `transportReady()`, and the
post-filtering assignment occurred after `transportReady()`. The code
now filters the attributes separately so they are updated before calling
`transportReady()`.
Original TSAN failure:
```
Read of size 4 at 0x0000cd44769c by thread T23:
#0 io.grpc.netty.NettyClientHandler.getAttributes()Lio/grpc/Attributes; NettyClientHandler.java:327
#1 io.grpc.netty.NettyClientTransport.getAttributes()Lio/grpc/Attributes; NettyClientTransport.java:363
#2 io.grpc.netty.NettyClientTransport.newStream(Lio/grpc/MethodDescriptor;Lio/grpc/Metadata;Lio/grpc/CallOptions;[Lio/grpc/ClientStreamTracer;)Lio/grpc/internal/ClientStream; NettyClientTransport.java:183
#3 io.grpc.internal.MetadataApplierImpl.apply(Lio/grpc/Metadata;)V MetadataApplierImpl.java:74
#4 io.grpc.auth.GoogleAuthLibraryCallCredentials$1.onSuccess(Ljava/util/Map;)V GoogleAuthLibraryCallCredentials.java:141
#5 com.google.auth.oauth2.OAuth2Credentials$FutureCallbackToMetadataCallbackAdapter.onSuccess(Lcom/google/auth/oauth2/OAuth2Credentials$OAuthValue;)V OAuth2Credentials.java:534
#6 com.google.auth.oauth2.OAuth2Credentials$FutureCallbackToMetadataCallbackAdapter.onSuccess(Ljava/lang/Object;)V OAuth2Credentials.java:525
...
Previous write of size 4 at 0x0000cd44769c by thread T24:
#0 io.grpc.netty.NettyClientHandler$FrameListener.onSettingsRead(Lio/netty/channel/ChannelHandlerContext;Lio/netty/handler/codec/http2/Http2Settings;)V NettyClientHandler.java:920
#1 io.netty.handler.codec.http2.DefaultHttp2ConnectionDecoder$FrameReadListener.onSettingsRead(Lio/netty/channel/ChannelHandlerContext;Lio/netty/handler/codec/http2/Http2Settings;)V DefaultHttp2ConnectionDecoder.java:515
...
```