gRPC example: add custom tags in client. (#407)

Do this by extending the current tagmap.

Author: g-easy

examples/grpc/BUILD

@@ -59,6 +59,9 @@ cc_binary(
         ":exporters",
         ":hello_cc_grpc",
         ":hello_cc_proto",
+        "//opencensus/tags",
+        "//opencensus/tags:context_util",
+        "//opencensus/tags:with_tag_map",
         "//opencensus/trace",
         "//opencensus/trace:context_util",
         "//opencensus/trace:with_span",

examples/grpc/hello_client.cc

@@ -25,6 +25,10 @@
 #include "examples/grpc/exporters.h"
 #include "examples/grpc/hello.grpc.pb.h"
 #include "examples/grpc/hello.pb.h"
+#include "opencensus/tags/context_util.h"
+#include "opencensus/tags/tag_key.h"
+#include "opencensus/tags/tag_map.h"
+#include "opencensus/tags/with_tag_map.h"
 #include "opencensus/trace/context_util.h"
 #include "opencensus/trace/sampler.h"
 #include "opencensus/trace/trace_config.h"
@@ -65,8 +69,19 @@ int main(int argc, char **argv) {
       "HelloClient", /*parent=*/nullptr, {&sampler});
   std::cout << "HelloClient span context is " << span.context().ToString()
             << "\n";
+
+  // Extend the current tag map.
+  // (in this example, there are no tags currently present, but in real code we
+  // wouldn't want to drop tags)
+  static const auto key = opencensus::tags::TagKey::Register("my_key");
+  std::vector<std::pair<opencensus::tags::TagKey, std::string>> tags(
+      opencensus::tags::GetCurrentTagMap().tags());
+  tags.emplace_back(key, "my_value");
+  opencensus::tags::TagMap tag_map(std::move(tags));
+
   {
     opencensus::trace::WithSpan ws(span);
+    opencensus::tags::WithTagMap wt(tag_map);
 
     // The client Span ends when ctx falls out of scope.
     grpc::ClientContext ctx;