Zipkin API 0.2.1

Zipkin API 0.2.1 fixes the result type of the span reporting endpoint to not imply a google empty type. This simplifies compiler configuration and dependencies needed to use the service.

For example, the following kotlin wire client no longer needs any google library dependencies as the result type is not a google type

    @WireRpc(
      path = "/zipkin.proto3.SpanService/Report",
      requestAdapter = "zipkin2.proto3.ListOfSpans#ADAPTER",
-      responseAdapter = "com.google.protobuf.Empty#ADAPTER"
+      responseAdapter = "zipkin2.proto3.ReportResponse#ADAPTER"
    )
-    suspend fun Report(request: ListOfSpans): Empty
+    suspend fun Report(request: ListOfSpans): ReportResponse
  }