Elasticsearch integration

[dd-caleb]

This is an Elasticsearch tracing integration. I swapped out the CallElasticsearch and CallElasticsearchAsync methods of the IPipeline interface which is used by Transport and a lot of other classes up stream, so all Nest functionality should get included as well.

One tricky thing here, CancellationToken is a value type with a single CancellationTokenSource field (an object type), so the method ends up receiving a CancellationTokenSource, hence why we do:

var cancellationToken = (cancellationTokenSource as CancellationTokenSource)?.Token ?? CancellationToken.None;