SDK: Implement Jaeger exporter

[pavolloffay]

Implement span exporter for Jaeger server.

hint: import from OpenCencus.

[jpkrohling]

I'm working on this one.

[jpkrohling]

We have a few options here.

1. The easiest for a first implementation is probably to add a dependency on the Jaeger Client for Java. A new OpenTelemetry SpanExporter would then create a Jaeger Span, which gets reported to Jaeger Agent/Collector via its own client;
2. For the long term, it might make sense to avoid a dependency to the Jaeger Client for Java. In the name of maintainability, we'd implement only gRPC/protobuf (preferred) or Thrift. Sending spans to the Jaeger Agent in protobuf format isn't supported yet, so, if we decide to use it already for the first implementation, we'd have support only for communication with the Jaeger Collector, bypassing the agent.

[jpkrohling]

cc @objectiser, @yurishkuro

[bogdandrutu]

Please for the moment put it in a separate gradle subproject under exporters/ directory, so that SDK does not depend on this.

[jpkrohling]

@bogdandrutu, what's the preferred approach? Should we attempt to avoid a dependency to the Jaeger Client, or should we reuse the Jaeger Client when possible (making sure to isolate this as its own module)?

[bogdandrutu]

I think we aim to become the official client for Jaeger or (recommended), so I think we need to directly implement the wire protocol. Also we already have all the batching logic etc, so no point in using the client unless they have a raw client that is a helper to just talk to the server.

[jpkrohling]

unless they have a raw client that is a helper to just talk to the server

There's a reporter+sender that could be directly used, but we might want to start supporting gRPC only, as we were already looking to phase out the Thrift channels. And for gRPC, it's quite easy to build the client. The biggest "problem" is to get the proto files, or their outcomes (generated client/models).