Add OcAgent Trace Exporter. (#355)

Fixes #196.

Author: panzhongxian

BUILD

@@ -150,3 +150,40 @@ switched_rules_by_language(
     cc = True,
     grpc = True,
 )
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
+    name = "io_bazel_rules_go",
+    sha256 = "9fb16af4d4836c8222142e54c9efa0bb5fc562ffc893ce2abeac3e25daead144",
+    urls = [
+        "https://storage.googleapis.com/bazel-mirror/github.com/bazelbuild/rules_go/releases/download/0.19.0/rules_go-0.19.0.tar.gz",
+        "https://github.com/bazelbuild/rules_go/releases/download/0.19.0/rules_go-0.19.0.tar.gz",
+    ],
+)
+
+load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")
+
+go_rules_dependencies()
+
+go_register_toolchains()
+
+# Needed by opencensus-proto.
+http_archive(
+    name = "grpc_java",
+    strip_prefix = "grpc-java-master",
+    urls = ["https://github.com/grpc/grpc-java/archive/master.zip"],
+)
+
+load("@grpc_java//:repositories.bzl", "grpc_java_repositories")
+
+grpc_java_repositories(
+    # Omit to avoid conflicts.
+    omit_com_google_protobuf = True,
+)
+
+http_archive(
+    name = "io_opencensus_proto",
+    strip_prefix = "opencensus-proto-master/src",
+    urls = ["https://github.com/census-instrumentation/opencensus-proto/archive/master.zip"],
+)

WORKSPACE

@@ -0,0 +1,56 @@
+# Copyright 2019, OpenCensus Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load("//opencensus:copts.bzl", "DEFAULT_COPTS", "TEST_COPTS")
+
+licenses(["notice"])  # Apache License 2.0
+
+package(default_visibility = ["//visibility:private"])
+
+# Libraries
+# ========================================================================= #
+
+cc_library(
+    name = "ocagent_exporter",
+    srcs = ["internal/ocagent_exporter.cc"],
+    hdrs = ["ocagent_exporter.h"],
+    copts = DEFAULT_COPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//opencensus/common:version",
+        "//opencensus/common/internal:hostname",
+        "//opencensus/common/internal/grpc:status",
+        "//opencensus/common/internal/grpc:with_user_agent",
+        "//opencensus/trace",
+        "@com_github_grpc_grpc//:grpc++",
+        "@com_google_absl//absl/base:core_headers",
+        "@com_google_absl//absl/memory",
+        "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/time",
+        "@io_opencensus_proto//opencensus/proto/agent/trace/v1:trace_service_grpc_cc",
+        "@io_opencensus_proto//opencensus/proto/agent/trace/v1:trace_service_proto_cc",
+    ],
+)
+
+cc_test(
+    name = "ocagent_exporter_test",
+    srcs = ["internal/ocagent_exporter_test.cc"],
+    copts = TEST_COPTS,
+    deps = [
+        ":ocagent_exporter",
+        "//opencensus/trace",
+        "@com_google_absl//absl/time",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

opencensus/exporters/trace/ocagent/BUILD

@@ -0,0 +1,57 @@
+# OpenCensus OcAgent Tracing Exporter
+
+The *OpenCensus OcAgent Tracing Exporter* is a tracing exporter that exports
+tracing to [OcAgent](https://opencensus.io/service/components/agent/).
+
+## Quickstart
+
+### Prerequisites
+
+Install OcAgent as the [doc](https://opencensus.io/service/components/agent/install). You can deploy an agent in the same Pod with service. Or deploy it on a remote machine.
+
+### Register the exporter
+
+Include:
+
+```c++
+#include "opencensus/exporters/trace/ocagent/ocagent_exporter.h"
+```
+
+Add a BUILD dependency on:
+
+```
+"@io_opencensus_cpp//exporters/trace/ocagent:ocagent_exporter",
+```
+
+In your application's initialization code, register the exporter:
+
+```c++
+opencensus::exporters::trace::OcAgentOptions opts;
+opts.address = "localhost:55678";
+OcAgentExporter::Register(std::move(opts));
+```
+
+#### Adding Spans to a Trace
+
+A trace consists of a tree of spans. You should build an `::opencensus::trace::StartSpanOptions` and then start an span.
+
+```c++
+::opencensus::trace::AlwaysSampler sampler;
+::opencensus::trace::StartSpanOptions opts = {&sampler};
+
+auto span1 = ::opencensus::trace::Span::StartSpan("Span1", nullptr, opts);
+```
+
+Set some fields of the span.
+
+```c++
+span1.AddAnnotation("Annotation1", {{"TestBool", true}});
+```
+
+When the Span is finish, call the `End()` function and after that the span will be exported.
+
+```c++
+span1.End();
+```
+
+For more usage details, you can find out in the test file `ocagent_exporter_test.cc`.