Add export service proto (#60)

We are experimentally working on a an export service
that will allow OpenCensus libraries to report collected
data to an external service for processing and exporting.

Some frameworks and ecosystems are now providing out-of-the-box
instrumentation by using OpenCensus but the user is still expected
to register an exporter in order to export data. This is a
problem during an incident. Even though our users can benefit
from having more diagnostics data coming out of services
already instrumented with OpenCensus, they have to modify
their code to register an exporter and redeploy. Asking our
users recompile and redeploy is not an ideal at an incident
time.

This experiment might be a possible solution to
eliminate this requirement.

Updates opencensus-specs/#72.

opencensus/proto/exporter/BUILD.bazel

@@ -0,0 +1,34 @@
+# Copyright 2018, 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.
+
+package(default_visibility = ["//visibility:public"])
+
+proto_library(
+    name = "exporter_proto",
+    srcs = ["exporter.proto"],
+    deps = [
+        "//opencensus/proto/trace:trace_proto",
+        "//opencensus/proto/stats/metrics:metrics_proto"
+    ],
+)
+
+cc_proto_library(
+    name = "exporter_proto_cc",
+    deps = [":exporter_proto"],
+)
+
+java_proto_library(
+    name = "exporter_proto_java",
+    deps = [":exporter_proto"],
+)

opencensus/proto/exporter/exporter.proto

@@ -0,0 +1,48 @@
+// Copyright 2018, 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.
+
+syntax = "proto3";
+
+// NOTE: This proto is experimental and is subject to change at this point.
+// Please do not use it at the moment.
+
+package opencensus.proto.exporter;
+
+import "opencensus/proto/trace/trace.proto";
+import "opencensus/proto/stats/metrics/metrics.proto";
+
+option java_multiple_files = true;
+option java_package = "io.opencensus.proto.exporter";
+option java_outer_classname = "ExporterProto";
+
+option go_package = "github.com/census-instrumentation/opencensus-proto/gen-go/exporterproto";
+
+message ExportSpanRequest {
+  repeated opencensus.proto.trace.Span spans = 1;
+}
+
+message ExportSpanResponse {
+}
+
+message ExportMetricsRequest {
+  repeated opencensus.proto.stats.metrics.Metric metrics = 1;
+}
+
+message ExportMetricsResponse {
+}
+
+service Export {
+  rpc ExportSpan (stream ExportSpanRequest) returns (stream ExportSpanResponse) {}
+  rpc ExportMetrics (stream ExportMetricsRequest) returns (stream ExportMetricsResponse) {}
+}