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/connection.go

@@ -0,0 +1,94 @@
+// 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"
+	"sync/atomic"
+	"time"
+)
+
+const (
+	sDisconnected int32 = 5 + iota
+	sConnected
+)
+
+func (ae *Exporter) setStateDisconnected() {
+	atomic.StoreInt32(&ae.connectionState, sDisconnected)
+	ae.disconnectedCh <- true
+}
+
+func (ae *Exporter) setStateConnected() {
+	atomic.StoreInt32(&ae.connectionState, sConnected)
+}
+
+func (ae *Exporter) connected() bool {
+	return atomic.LoadInt32(&ae.connectionState) == sConnected
+}
+
+const defaultConnReattemptPeriod = 10 * time.Second
+
+func (ae *Exporter) indefiniteBackgroundConnection() error {
+	defer func() {
+		ae.backgroundConnectionDoneCh <- true
+	}()
+
+	connReattemptPeriod := ae.reconnectionPeriod
+	if connReattemptPeriod <= 0 {
+		connReattemptPeriod = defaultConnReattemptPeriod
+	}
+
+	// No strong seeding required, nano time can
+	// already help with pseudo uniqueness.
+	rng := rand.New(rand.NewSource(time.Now().UnixNano() + rand.Int63n(1024)))
+
+	// maxJitter: 1 + (70% of the connectionReattemptPeriod)
+	maxJitter := int64(1 + 0.7*float64(connReattemptPeriod))
+
+	for {
+		// Otherwise these will be the normal scenarios to enable
+		// reconnections if we trip out.
+		// 1. If we've stopped, return entirely
+		// 2. Otherwise block until we are disconnected, and
+		//    then retry connecting
+		select {
+		case <-ae.stopCh:
+			return errStopped
+
+		case <-ae.disconnectedCh:
+			// Normal scenario that we'll wait for
+		}
+
+		if err := ae.connect(); err == nil {
+			ae.setStateConnected()
+		} else {
+			ae.setStateDisconnected()
+		}
+
+		// Apply some jitter to avoid lockstep retrials of other
+		// agent-exporters. Lockstep retrials could result in an
+		// innocent DDOS, by clogging the machine's resources and network.
+		jitter := time.Duration(rng.Int63n(maxJitter))
+		<-time.After(connReattemptPeriod + jitter)
+	}
+}
+
+func (ae *Exporter) connect() error {
+	cc, err := ae.dialToAgent()
+	if err != nil {
+		return err
+	}
+	return ae.enableConnectionStreams(cc)
+}