Add ai3/go-common/tracing and deps

vendor/go.opencensus.io/stats/units.go

+// 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 stats
+
+// Units are encoded according to the case-sensitive abbreviations from the
+// Unified Code for Units of Measure: http://unitsofmeasure.org/ucum.html
+const (
+	UnitNone          = "1" // Deprecated: Use UnitDimensionless.
+	UnitDimensionless = "1"
+	UnitBytes         = "By"
+	UnitMilliseconds  = "ms"
+)