Make opencensus Stackdriver exporter respects initial_metadata option…

bazel/repository_locations.bzl

@@ -388,10 +388,10 @@ DEPENDENCY_REPOSITORIES = dict(
         use_category = ["other"],
     ),
     io_opencensus_cpp = dict(
-        sha256 = "193ffb4e13bd7886757fd22b61b7f7a400634412ad8e7e1071e73f57bedd7fc6",
-        strip_prefix = "opencensus-cpp-04ed0211931f12b03c1a76b3907248ca4db7bc90",
-        # 2020-03-24
-        urls = ["https://github.com/census-instrumentation/opencensus-cpp/archive/04ed0211931f12b03c1a76b3907248ca4db7bc90.tar.gz"],
+        sha256 = "12ff300fa804f97bd07e2ff071d969e09d5f3d7bbffeac438c725fa52a51a212",
+        strip_prefix = "opencensus-cpp-7877337633466358ed680f9b26967da5b310d7aa",
+        # 2020-06-01
+        urls = ["https://github.com/census-instrumentation/opencensus-cpp/archive/7877337633466358ed680f9b26967da5b310d7aa.tar.gz"],
         use_category = ["observability"],
         cpe = "N/A",
     ),

source/extensions/tracers/opencensus/opencensus_tracer_impl.cc

@@ -281,7 +281,17 @@ Driver::Driver(const envoy::config::trace::v3::OpenCensusConfig& oc_config,
         stackdriver_service.mutable_google_grpc()->set_target_uri(GoogleStackdriverTraceAddress);
       }
       auto channel = Envoy::Grpc::GoogleGrpcUtils::createChannel(stackdriver_service, api);
+      // TODO(bianpengyuan): add tests for trace_service_stub and initial_metadata options with mock
+      // stubs.
       opts.trace_service_stub = ::google::devtools::cloudtrace::v2::TraceService::NewStub(channel);
+      const auto& initial_metadata = stackdriver_service.initial_metadata();
+      if (!initial_metadata.empty()) {
+        opts.prepare_client_context = [initial_metadata](grpc::ClientContext* ctx) {
+          for (const auto& metadata : initial_metadata) {
+            ctx->AddMetadata(metadata.key(), metadata.value());
+          }
+        };
+      }
 #else
       throw EnvoyException("Opencensus tracer: cannot handle stackdriver google grpc service, "
                            "google grpc is not built in.");

test/extensions/tracers/opencensus/config_test.cc

@@ -322,6 +322,9 @@ TEST(OpenCensusTracerConfigTest, OpenCensusHttpTracerStackdriverGrpc) {
         google_grpc:
           target_uri: 127.0.0.1:55678
           stat_prefix: test
+        initial_metadata:
+        - key: foo
+          value: bar
   )EOF";
 
   envoy::config::trace::v3::Tracing configuration;