Update go dependencies

vendor/go.opencensus.io/exemplar/exemplar.go

@@ -0,0 +1,78 @@
+// 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 exemplar implements support for exemplars. Exemplars are additional
+// data associated with each measurement.
+//
+// Their purpose it to provide an example of the kind of thing
+// (request, RPC, trace span, etc.) that resulted in that measurement.
+package exemplar
+
+import (
+	"context"
+	"time"
+)
+
+const (
+	KeyTraceID   = "trace_id"
+	KeySpanID    = "span_id"
+	KeyPrefixTag = "tag:"
+)
+
+// Exemplar is an example data point associated with each bucket of a
+// distribution type aggregation.
+type Exemplar struct {
+	Value       float64     // the value that was recorded
+	Timestamp   time.Time   // the time the value was recorded
+	Attachments Attachments // attachments (if any)
+}
+
+// Attachments is a map of extra values associated with a recorded data point.
+// The map should only be mutated from AttachmentExtractor functions.
+type Attachments map[string]string
+
+// AttachmentExtractor is a function capable of extracting exemplar attachments
+// from the context used to record measurements.
+// The map passed to the function should be mutated and returned. It will
+// initially be nil: the first AttachmentExtractor that would like to add keys to the
+// map is responsible for initializing it.
+type AttachmentExtractor func(ctx context.Context, a Attachments) Attachments
+
+var extractors []AttachmentExtractor
+
+// RegisterAttachmentExtractor registers the given extractor associated with the exemplar
+// type name.
+//
+// Extractors will be used to attempt to extract exemplars from the context
+// associated with each recorded measurement.
+//
+// Packages that support exemplars should register their extractor functions on
+// initialization.
+//
+// RegisterAttachmentExtractor should not be called after any measurements have
+// been recorded.
+func RegisterAttachmentExtractor(e AttachmentExtractor) {
+	extractors = append(extractors, e)
+}
+
+// NewFromContext extracts exemplars from the given context.
+// Each registered AttachmentExtractor (see RegisterAttachmentExtractor) is called in an
+// unspecified order to add attachments to the exemplar.
+func AttachmentsFromContext(ctx context.Context) Attachments {
+	var a Attachments
+	for _, extractor := range extractors {
+		a = extractor(ctx, a)
+	}
+	return a
+}