Add tags package based on the specs.

src/opencensus/proto/stats/v1/BUILD.bazel

@@ -20,6 +20,7 @@ proto_library(
     name = "stats_proto",
     srcs = ["stats.proto"],
     deps = [
+        "//opencensus/proto/tags/v1:tags_proto",
         "@com_google_protobuf//:duration_proto",
         "@com_google_protobuf//:timestamp_proto",
     ],
@@ -37,9 +38,10 @@ java_proto_library(
 
 go_proto_library(
     name = "stats_proto_go",
-    proto = ":stats_proto",
     importpath = "github.com/census-instrumentation/opencensus-proto/gen-go/stats/v1",
+    proto = ":stats_proto",
     deps = [
+        "//opencensus/proto/tags/v1:tags_proto_go",
         "@com_github_golang_protobuf//ptypes/timestamp:go_default_library",
     ],
 )

src/opencensus/proto/stats/v1/stats.proto

@@ -17,20 +17,14 @@ syntax = "proto3";
 package opencensus.proto.stats.v1;
 
 import "google/protobuf/timestamp.proto";
+import "opencensus/proto/tags/v1/tags.proto";
 
 option go_package = "github.com/census-instrumentation/opencensus-proto/gen-go/stats/v1";
 
 option java_multiple_files = true;
 option java_package = "io.opencensus.proto.stats.v1";
 option java_outer_classname = "StatsProto";
 
-// TODO(bdrutu): Consider if this should be moved to a "tags" directory to match the API structure.
-message Tag {
-  string key = 1;
-  string value = 2;
-}
-
-// Measure .
 message Measure {
   // A string by which the measure will be referred to, e.g. "rpc_server_latency". Names MUST be
   // unique within the library.
@@ -119,7 +113,7 @@ message DistributionAggregation {
 
 // Describes a data point to be collected for a Measure.
 message Measurement {
-  repeated Tag tags = 1;
+  opencensus.proto.tags.v1.TagMap tag_map = 1;
 
   // The name of the measure to which the value is applied.
   string measure_name = 2;

src/opencensus/proto/tags/v1/BUILD.bazel

@@ -0,0 +1,41 @@
+# 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.
+
+package(default_visibility = ["//visibility:public"])
+
+load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
+
+proto_library(
+    name = "tags_proto",
+    srcs = ["tags.proto"],
+)
+
+cc_proto_library(
+    name = "tags_proto_cc",
+    deps = [":tags_proto"],
+)
+
+java_proto_library(
+    name = "tags_proto_java",
+    deps = [":tags_proto"],
+)
+
+go_proto_library(
+    name = "tags_proto_go",
+    importpath = "github.com/census-instrumentation/opencensus-proto/gen-go/tags/v1",
+    proto = ":tags_proto",
+    deps = [
+        "@com_github_golang_protobuf//ptypes/timestamp:go_default_library",
+    ],
+)

src/opencensus/proto/tags/v1/tags.proto

@@ -0,0 +1,79 @@
+// 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.
+
+syntax = "proto3";
+
+package opencensus.proto.tags.v1;
+
+option go_package = "github.com/census-instrumentation/opencensus-proto/gen-go/tags/v1";
+
+option java_multiple_files = true;
+option java_package = "io.opencensus.proto.tags.v1";
+option java_outer_classname = "TagsProto";
+
+// TagScope is used to determine the scope of a Tag.
+message TagScope {
+  enum Scope {
+    // Unknown type.
+    TYPE_UNSPECIFIED = 0;
+    // Tag with Local scope are used within the process it created.
+    LOCAL_SCOPE = 1;
+    // A tag is created with the Request scope then it is propagated across
+    // process boundaries subject to outgoing and incoming (on remote side)
+    // filter criteria.
+    REQUEST_SCOPE = 2;
+  }
+
+  // This field is required.
+  Scope scope = 1;
+}
+
+// TagKey is the name of the Tag.
+message TagKey {
+  // Restrictions:
+  //   * MUST contain only printable ASCII (codes between 32 and 126 inclusive)
+  //   * MUST have length greater than zero and less than 256.
+  //   * MUST not be empty.
+  //
+  // This field is required.
+  string value = 1;
+}
+
+// TagValue is the value associated with a Tag.
+message TagValue {
+  // It MUST contain only printable ASCII (codes between 32 and 126)
+  //
+  // This field is required.
+  string value = 1;
+}
+
+// A Tag consists of TagScope, TagKey, and TagValue.
+message Tag {
+  // This field is required.
+  TagKey key = 1;
+
+  // This field is required.
+  TagValue value = 2;
+
+  // This field is optional, if not present the default scope is REQUEST_SCOPE
+  // (the motivation is backwards compatibility).
+  TagScope scope = 3;
+}
+
+// TagMap is an abstract data type that represents collection of tags. i.e.,
+// each key is associated with exactly one value.
+message TagMap {
+  // Not a map becuase the proto3 map does not allow a message as the key.
+  repeated Tag tags = 1;
+}