Add instrumentation for Guava ListenableFutures context propagation

[tylerbenson]

This helps to ensure the current span when the callback is registered is accessible when the callback executes.

[richardstartin]

Can you explain how this helps to ensure that the current span when the callback is registered is accessible when the callback executes, as if to a five year old?

[tylerbenson]

This follows the same principle as our regular executor instrumentation. When a runnable is added as a listener, we capture the current context (span) and attach it to the runnable. Later, when the runnable is executed, it finds that context and restores it into the current thread, which is potentially different from the original thread.