Add trace_config.proto. (#80)

* Add trace_config.proto

* Combine AlwaysSampler and NeverSampler.

* Make trace_config_proto a separate library.

opencensus/proto/trace/BUILD.bazel

@@ -23,12 +23,27 @@ proto_library(
     ],
 )
 
+proto_library(
+    name = "trace_config_proto",
+    srcs = ["trace_config.proto"],
+)
+
 cc_proto_library(
     name = "trace_proto_cc",
     deps = [":trace_proto"],
 )
 
+cc_proto_library(
+    name = "trace_config_proto_cc",
+    deps = [":trace_config_proto"],
+)
+
 java_proto_library(
     name = "trace_proto_java",
     deps = [":trace_proto"],
 )
+
+java_proto_library(
+    name = "trace_config_proto_java",
+    deps = [":trace_config_proto"],
+)

opencensus/proto/trace/trace_config.proto

@@ -0,0 +1,61 @@
+// 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";
+
+package opencensus.proto.trace;
+
+option java_multiple_files = true;
+option java_package = "io.opencensus.proto.trace";
+option java_outer_classname = "TraceConfigProto";
+
+option go_package = "github.com/census-instrumentation/opencensus-proto/gen-go/traceconfigproto";
+
+// Global configuration of the trace service.
+message TraceConfig {
+
+  // The global default sampler used to make decisions on span sampling.
+  oneof sampler {
+    ProbabilitySampler probability_sampler = 1;
+
+    ConstantSampler constant_sampler = 2;
+
+    RateLimitingSampler rate_limiting_sampler = 3;
+  }
+
+  // TODO(songya): add more fields.
+}
+
+// Sampler that tries to uniformly sample traces with a given probability.
+// The probability of sampling a trace is equal to that of the specified probability.
+message ProbabilitySampler {
+
+  // The desired probability of sampling. Must be within [0.0, 1.0].
+  double samplingProbability = 1;
+}
+
+// Sampler that makes a constant decision (either always "yes" or always "no")
+// on span sampling.
+message ConstantSampler {
+
+  // Whether spans should be always sampled, or never sampled.
+  bool decision = 1;
+}
+
+// Sampler that tries to sample with a rate per time window.
+message RateLimitingSampler {
+
+  // Rate per second.
+  int64 qps = 1;
+}