Add example of build system.

WORKSPACE

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+# Copyright 2019, OpenTelemetry 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.
+
+workspace(name = "io_opentelemetry_cpp")
+
+# GoogleTest framework.
+# Only needed for tests, not to build the OpenTelemetry library.
+http_archive(
+    name = "com_google_googletest",
+    strip_prefix = "googletest-master",
+    urls = ["https://github.com/google/googletest/archive/master.zip"],
+)

api/include/opentelemetry/trace/BUILD

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+# Copyright 2019, OpenTelemetry 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.
+
+package(default_visibility = ["//visibility:private"])
+
+cc_library(
+    name = "trace",
+    hdrs = ["span_id.h"],
+    visibility = ["//visibility:public"],
+    includes = ["../.."],
+)

api/src/opentelemetry/trace/BUILD

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+# Copyright 2019, OpenTelemetry 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.
+
+package(default_visibility = ["//visibility:private"])
+
+cc_library(
+    name = "trace",
+    srcs = ["span_id.cc"],
+    visibility = ["//visibility:public"],
+    deps = ["@io_opentelemetry_cpp//api/include/opentelemetry/trace"],
+)

examples/README.md

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+The examples/ directory is a separate bazel workspace, to show how
+to build libraries and binaries that use the OpenTelemetry library.
+
+```
+cd examples/
+bazel build //helloworld
+```

examples/WORKSPACE

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+# Copyright 2019, OpenTelemetry 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.
+
+workspace(name = "io_opentelemetry_cpp_examples")
+
+# OpenTelemetry should be imported via http_archive.
+local_repository(
+    name = "io_opentelemetry_cpp",
+    path = "../",
+)

examples/helloworld/BUILD

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+# Copyright 2019, OpenTelemetry 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.
+
+cc_binary(
+    name = "helloworld",
+    srcs = ["helloworld.cc"],
+    deps = [
+        "@io_opentelemetry_cpp//api/include/opentelemetry/trace",
+        "@io_opentelemetry_cpp//api/src/opentelemetry/trace",
+    ],
+)

examples/helloworld/helloworld.cc

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+// Copyright 2019, OpenTelemetry 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.
+
+#include <opentelemetry/trace/span_id.h>
+
+#include <iostream>
+
+int main() {
+  opentelemetry::trace::SpanId span_id;
+  std::cout << "Span ID: " << span_id.ToHex() << "\n";
+}