Add support for tracing via OpenCencus

This adds a few flags for configuring the tracer.
Includes support for jaeger tracing (built into OC).

vendor/go.opencensus.io/stats/doc.go

@@ -0,0 +1,55 @@
+// Copyright 2017, 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 stats contains support for OpenCensus stats recording.
+
+OpenCensus allows users to create typed measures, record measurements,
+aggregate the collected data, and export the aggregated data.
+
+Measures
+
+A measure represents a type of metric to be tracked and recorded.
+For example, latency, request Mb/s, and response Mb/s are measures
+to collect from a server.
+
+Each measure needs to be registered before being used. Measure
+constructors such as Int64 and Float64 automatically
+register the measure by the given name. Each registered measure needs
+to be unique by name. Measures also have a description and a unit.
+
+Libraries can define and export measures for their end users to
+create views and collect instrumentation data.
+
+Recording measurements
+
+Measurement is a data point to be collected for a measure. For example,
+for a latency (ms) measure, 100 is a measurement that represents a 100ms
+latency event. Users collect data points on the existing measures with
+the current context. Tags from the current context are recorded with the
+measurements if they are any.
+
+Recorded measurements are dropped immediately if user is not aggregating
+them via views. Users don't necessarily need to conditionally enable/disable
+recording to reduce cost. Recording of measurements is cheap.
+
+Libraries can always record measurements, and end-users can later decide
+on which measurements they want to collect by registering views. This allows
+libraries to turn on the instrumentation by default.
+*/
+package stats // import "go.opencensus.io/stats"
+
+// TODO(acetechnologist): Add a link to the language independent OpenCensus
+// spec when it is available.