Rework the metric_descriptor type (#36)

Rework the metric_descriptor to separate the description of the shape of the metric from the attributes used to produce values for the metric at run time; the mapping from attribute to label value is described in the aspect's config in the mixer at //pkg/aspect/config/metrics.proto

Author: ZackButcher

.gitignore

@@ -27,3 +27,5 @@
 *.exe
 *.out
 *.app
+
+.idea/

mixer/v1/config/descriptor/label_descriptor.proto

@@ -0,0 +1,31 @@
+// Copyright 2017 Google Inc.
+//
+// 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 istio.mixer.v1.config.descriptor;
+
+import "mixer/v1/config/descriptor/value_type.proto";
+
+// A `LabelDescriptor` describes the schema of a label (or dimension) used in other descriptors.
+message LabelDescriptor {
+    // The name of this descriptor, referenced from other descriptors.
+    string name = 1;
+
+    // An optional human-readable description of the attribute's purpose.
+    string description = 2;
+
+    // The type of data carried by this label
+    ValueType value_type = 3;
+}

mixer/v1/config/descriptor/metric_descriptor.proto

@@ -16,32 +16,50 @@ syntax = "proto3";
 
 package istio.mixer.v1.config.descriptor;
 
+import "mixer/v1/config/descriptor/label_descriptor.proto";
+import "mixer/v1/config/descriptor/value_type.proto";
+
 // Defines a metric type and its schema.
 //
-// A given metric is dimensioned by a set of attributes. A given metric holds a
-// unique value for potentially any combination of these attributes.
+// A metric is dimensioned by a set of labels whose values are derived at runtime from attributes.
+// A given metric holds a unique value for potentially any combination of these dimensions.
+//
+// The following is an example descriptor for a metric capturing the number of RPCs served, dimensioned
+// by the method being called and response code returned by the server:
+//
+//    metric_descriptor:
+//      name: "response_code"
+//      kind: COUNTER
+//      value: I64
+//      labels:
+//        name: api_method
+//        value_type: STRING
+//      labels:
+//        name: response_code
+//        value_type: INT64
+//
+// To actually report metrics at run time a mapping from attributes to a metric's labels must be provided.
+// This is provided in the service's config; using our above descriptor we might describe the metric as:
+//
+//    metric:
+//      descriptor: "response_code" # must match metric_descriptor.name
+//      value: $requestCount        # Istio expression syntax for the attribute named "request_count"
+//      labels:
+//        # either the attribute named 'apiMethod' or the literal string 'unknown'; must eval to a string
+//        api_method: $apiMethod | "unknown"
+//        # either the attribute named 'responseCode' or the literal int64 500; must eval to an int64
+//        response_code: $responseCode | 500
+//      
 message MetricDescriptor {
-  // The name of this descriptor.
+  // The name of this descriptor. This is used to refer to this descriptor in other contexts.
   string name = 1;
 
-  // The name of the attribute that supplies the value for metrics
-  // of this type.
-  string value_attribute = 2;
-
-  // The set of attributes that are necessary to describe a specific value cell
-  // for a metric of this type.
-  repeated string attributes = 3;
-
-  // Whether the metric records instantaneous values, changes to a value, etc.
-  MetricKind metric_kind = 4;
-
-  // An optional description of the metric, which should be used as the
-  // documentation of the metric.
-  string description = 5;
-
   // An optional concise name for the metric, which can be displayed in user interfaces.
   // Use sentence case without an ending period, for example "Request count".
-  string display_name = 6;
+  string display_name = 2;
+
+  // An optional description of the metric, which should be used as the documentation for the metric.
+  string description = 3;
 
   // The kind of measurement. It describes how the data is recorded.
   enum MetricKind {
@@ -55,4 +73,14 @@ message MetricDescriptor {
     // For example, the number of API requests.
     COUNTER = 2;
   }
+  // Whether the metric records instantaneous values, changes to a value, etc.
+  MetricKind kind = 4;
+
+  // The type of data this metric records.
+  ValueType value = 5;
+
+  // Labels that dimension the data recorded by this metric. The metric definition allows the user to
+  // map attribute expressions to actual values for these labels at run time; the result of the evaluation
+  // must be of the type described by the kind for each label.
+  repeated LabelDescriptor labels = 6;
 }