Add ocagent exporter

ocagent allows users to send traces out of VK to a "standard"  external forwarder
(standard as in this is a format/protocol defined in the opencensus project).

This allows uers to implement metrics for whatever backend they want
without having to add it into VK directly.

vendor/contrib.go.opencensus.io/exporter/ocagent/common.go

@@ -0,0 +1,38 @@
+// 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 ocagent
+
+import (
+	"math/rand"
+	"time"
+)
+
+var randSrc = rand.New(rand.NewSource(time.Now().UnixNano()))
+
+// retries function fn upto n times, if fn returns an error lest it returns nil early.
+// It applies exponential backoff in units of (1<<n) + jitter microsends.
+func nTriesWithExponentialBackoff(nTries int64, timeBaseUnit time.Duration, fn func() error) (err error) {
+	for i := int64(0); i < nTries; i++ {
+		err = fn()
+		if err == nil {
+			return nil
+		}
+		// Backoff for a time period with a pseudo-random jitter
+		jitter := time.Duration(randSrc.Float64()*100) * time.Microsecond
+		ts := jitter + ((1 << uint64(i)) * timeBaseUnit)
+		<-time.After(ts)
+	}
+	return err
+}