Change apm-spring-cloud-gateway plugin witness class.

[weixiang1862]

My spring cloud gateway version is 2.2.5.RELEASE, and I active both optional apm-spring-cloud-gateway 2.0.x, 2.1.x and 3.x plugins, then I found this error, seems 2.1.x plugin and 3.x plugin all actived because 3.x and 2.1.x use the same witness class org.springframework.cloud.gateway.config.GatewayEnvironmentPostProcessor.

I konw it is stupid to active both 2.0.x, 2.1.x and 3.x optional plugins, but if we change 3.x plugin witness class to org.springframework.cloud.gateway.config.GatewayLoadBalancerProperties, and add another witness class org.springframework.cloud.gateway.filter.LoadBalancerClientFilter to 2.1.x plugin, we can prevent this error even they all actived.

The org.springframework.cloud.gateway.filter.LoadBalancerClientFilter exist in 2.1.x and 2.2.x.
The org.springframework.cloud.gateway.config.GatewayLoadBalancerProperties exist in both 3.x and latest 4.x, I think it is a good choice before 4.x plugin born.

java.lang.ClassCastException: org.apache.skywalking.apm.plugin.spring.cloud.gateway.v21x.define.EnhanceObjectCache cannot be cast to org.apache.skywalking.apm.plugin.spring.cloud.gateway.v3x.define.EnhanceObjectCache
	at org.apache.skywalking.apm.plugin.spring.cloud.gateway.v3x.HttpClientFinalizerResponseConnectionInterceptor.lambda$afterMethod$2(HttpClientFinalizerResponseConnectionInterceptor.java:82)
	at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.runFinally(FluxDoFinally.java:156)
	at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onComplete(FluxDoFinally.java:139)
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:252)
	at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2346)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:143)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:182)
	at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99)
	at reactor.core.publisher.FluxRetryWhen$RetryWhenMainSubscriber.onNext(FluxRetryWhen.java:162)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:156)
	at reactor.netty.http.client.HttpClientConnect$HttpIOHandlerObserver.onStateChange(HttpClientConnect.java:431)
	at reactor.netty.ReactorNetty$CompositeConnectionObserver.onStateChange(ReactorNetty.java:514)
	at reactor.netty.resources.PooledConnectionProvider$DisposableAcquire.onStateChange(PooledConnectionProvider.java:540)
	at reactor.netty.resources.PooledConnectionProvider$PooledConnection.onStateChange(PooledConnectionProvider.java:427)
	at reactor.netty.http.client.HttpClientOperations.onInboundNext(HttpClientOperations.java:574)
	at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:96)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436)
	at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:324)
	at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:311)
	at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:425)
	at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:276)
	at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
	at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:792)
	at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:475)
	at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:378)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:748)

[wu-sheng]

You could add new witness classes if you want. It is improving for sure.

[weixiang1862]

Ok, thanks. I will submit a pr after some tests.

[weixiang1862]

fix by apache/skywalking-java#610.