Add public GetSpanFromServerContext() API. (#60)

Author: g-easy

opencensus/plugins/grpc/grpc_plugin.h

@@ -16,6 +16,11 @@
 #define OPENCENSUS_PLUGINS_GRPC_GRPC_PLUGIN_H_
 
 #include "absl/strings/string_view.h"
+#include "opencensus/trace/span.h"
+
+namespace grpc {
+class ServerContext;
+}
 
 namespace opencensus {
 
@@ -24,6 +29,9 @@ namespace opencensus {
 // defined below.
 void RegisterGrpcPlugin();
 
+// Returns the tracing Span for the current RPC.
+opencensus::trace::Span GetSpanFromServerContext(grpc::ServerContext* context);
+
 // The tag key for the RPC method and status, set for all values recorded for
 // the following measures.
 extern const absl::string_view kMethodTagKey;

opencensus/plugins/grpc/internal/grpc_plugin.cc

@@ -13,11 +13,14 @@
 // limitations under the License.
 
 #include "opencensus/plugins/grpc/grpc_plugin.h"
+
+#include <grpc++/server_context.h>
+
 #include "opencensus/plugins/grpc/internal/channel_filter.h"
 #include "opencensus/plugins/grpc/internal/client_filter.h"
 #include "opencensus/plugins/grpc/internal/measures.h"
 #include "opencensus/plugins/grpc/internal/server_filter.h"
-#include "src/core/lib/surface/channel_init.h"
+#include "opencensus/trace/span.h"
 
 namespace opencensus {
 
@@ -52,6 +55,11 @@ void RegisterGrpcPlugin() {
   RpcServerResponseCount();
 }
 
+opencensus::trace::Span GetSpanFromServerContext(grpc::ServerContext* context) {
+  return reinterpret_cast<const CensusContext*>(context->census_context())
+      ->Span();
+}
+
 // These measure definitions should be kept in sync across opencensus
 // implementations--see
 // https://github.com/census-instrumentation/opencensus-java/blob/master/contrib/grpc_metrics/src/main/java/io/opencensus/contrib/grpc/metrics/RpcMeasureConstants.java.