fix npe in guava eventbus plugin

[xzyJavaX]

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes [Bug] Guava-eventbus-plugin throw NPE. skywalking#10967
• Update the CHANGES log.

[wu-sheng]

I think we need to explain why the context could be either active or not?

[wu-sheng]

@xzyJavaX Any update?

[xzyJavaX]

When start a new thread to call asyncEventBus.post(testEvent), the context will be inactive, this situation has no context.

[wu-sheng]

So, in this case, the posting message is not traced? Isn't this a bug rather than fix NPE?

[lujiajing1126]

So, in this case, the posting message is not traced? Isn't this a bug rather than fix NPE?

I suppose It is traced but orphan. When calling ContextManager.capture to capture the state of TracingContext in the current thread, it throws NPE since the TracingContext does not exist in a new thread.

I think it is users' responsibility to propagate tracing context in this case, e.g. use @TraceCrossThread.

This PR can only avoid NPE to be thrown.

[wu-sheng]

Make sense. Please fix the UT failing.