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/options.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 ocagent
+
+import "time"
+
+const (
+	DefaultAgentPort uint16 = 55678
+	DefaultAgentHost string = "localhost"
+)
+
+type ExporterOption interface {
+	withExporter(e *Exporter)
+}
+
+type insecureGrpcConnection int
+
+var _ ExporterOption = (*insecureGrpcConnection)(nil)
+
+func (igc *insecureGrpcConnection) withExporter(e *Exporter) {
+	e.canDialInsecure = true
+}
+
+// WithInsecure disables client transport security for the exporter's gRPC connection
+// just like grpc.WithInsecure() https://godoc.org/google.golang.org/grpc#WithInsecure
+// does. Note, by default, client security is required unless WithInsecure is used.
+func WithInsecure() ExporterOption { return new(insecureGrpcConnection) }
+
+type addressSetter string
+
+func (as addressSetter) withExporter(e *Exporter) {
+	e.agentAddress = string(as)
+}
+
+var _ ExporterOption = (*addressSetter)(nil)
+
+// WithAddress allows one to set the address that the exporter will
+// connect to the agent on. If unset, it will instead try to use
+// connect to DefaultAgentHost:DefaultAgentPort
+func WithAddress(addr string) ExporterOption {
+	return addressSetter(addr)
+}
+
+type serviceNameSetter string
+
+func (sns serviceNameSetter) withExporter(e *Exporter) {
+	e.serviceName = string(sns)
+}
+
+var _ ExporterOption = (*serviceNameSetter)(nil)
+
+// WithServiceName allows one to set/override the service name
+// that the exporter will report to the agent.
+func WithServiceName(serviceName string) ExporterOption {
+	return serviceNameSetter(serviceName)
+}
+
+type reconnectionPeriod time.Duration
+
+func (rp reconnectionPeriod) withExporter(e *Exporter) {
+	e.reconnectionPeriod = time.Duration(rp)
+}
+
+func WithReconnectionPeriod(rp time.Duration) ExporterOption {
+	return reconnectionPeriod(rp)
+}