Fix netty issue

[trask]

App was failing due to:

java.util.NoSuchElementException: io.opentelemetry.javaagent.instrumentation.netty.v4.common.client.NettySslInstrumentationHandler
	at io.netty.channel.DefaultChannelPipeline.getContextOrDie(DefaultChannelPipeline.java:1073)
	at io.netty.channel.DefaultChannelPipeline.addAfter(DefaultChannelPipeline.java:302)
	at io.netty.channel.DefaultChannelPipeline.addAfter(DefaultChannelPipeline.java:290)

because the NettySslInstrumentationHandler was removing itself.

I'm not sure why it was removing itself, but tests still pass without that code, and my test app no longer fails.

[mateuszrzeszutek]

Is there an easy way to reproduce this? I suspect that the real error might be around here: https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/instrumentation/netty/netty-4.1/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v4_1/NettyChannelPipelineInstrumentation.java#L128
Where we might call addAfter to add a handler after NettySslInstrumentationHandler is no longer there

Removing the NettySslInstrumentationHandler kinda makes sense:

• the first remove method call is just a sanity check that prevents it from being called if the event class is not on the classpath (should never happen really)
• the second remove call is there to prevent duplicate ending of the SSL span - this is more or less defensive code, since I'm not sure whether it is even possible to get two of these events. I think we could replace this with setting context = null after ending the span the first time.

Reactor-netty's SslProvider.SslReadHandler does a similar thing, so while deleting the remove() calls might fix the issue, I think we should fix the underlying issue is possible -- it might happen again in some other context.

[trask]

@mateuszrzeszutek thx! that makes a lot of sense, I pushed a new fix

[laurit]

Perhaps here should be a comment that the same virtual field is used elsewhere. Currently it is not obvious what it is for because in this file this virtual field is only set to null.