gRPC example: show current span context and tags in server. (#413)

Author: g-easy

examples/grpc/BUILD

@@ -81,7 +81,10 @@ cc_binary(
         ":hello_cc_grpc",
         ":hello_cc_proto",
         "//opencensus/exporters/stats/prometheus:prometheus_exporter",
+        "//opencensus/tags",
+        "//opencensus/tags:context_util",
         "//opencensus/trace",
+        "//opencensus/trace:context_util",
         "@com_github_grpc_grpc//:grpc++",
         "@com_github_grpc_grpc//:grpc_opencensus_plugin",
         "@com_github_jupp0r_prometheus_cpp//pull",

examples/grpc/hello_server.cc

@@ -14,14 +14,14 @@
 
 // Example RPC server using gRPC.
 
+#include <grpcpp/grpcpp.h>
+#include <grpcpp/opencensus.h>
+
 #include <cstdlib>
 #include <iostream>
 #include <memory>
 #include <string>
 
-#include <grpcpp/grpcpp.h>
-#include <grpcpp/opencensus.h>
-
 #include "absl/strings/escaping.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
@@ -30,6 +30,9 @@
 #include "examples/grpc/hello.pb.h"
 #include "opencensus/exporters/stats/prometheus/prometheus_exporter.h"
 #include "opencensus/stats/stats.h"
+#include "opencensus/tags/context_util.h"
+#include "opencensus/tags/tag_map.h"
+#include "opencensus/trace/context_util.h"
 #include "opencensus/trace/sampler.h"
 #include "opencensus/trace/span.h"
 #include "opencensus/trace/trace_config.h"
@@ -89,8 +92,13 @@ class HelloServiceImpl final : public HelloService::Service {
         {{LettersMeasure(), request->name().size()}},
         {{CaseKey(), isupper(request->name()[0]) ? "upper" : "lower"}});
     // Give feedback on stderr.
-    std::cerr << "SayHello RPC handled.\n";
-    std::cerr << "  Metadata:\n";
+    std::cerr << "SayHello RPC handler:\n";
+    std::cerr << "  Current context: "
+              << opencensus::trace::GetCurrentSpan().context().ToString()
+              << "\n";
+    std::cerr << "  Current tags: "
+              << opencensus::tags::GetCurrentTagMap().DebugString() << "\n";
+    std::cerr << "  gRPC metadata:\n";
     auto metadata = context->client_metadata();
     for (const auto &mdpair : metadata) {
       std::cerr << "    \"" << absl::CEscape(ToStringView(mdpair.first))