[VOL-2193] Create mocks for Kafka Client and Etcd

This commit consists of:
1) A kafka client mock that implements the kafka client interface
under voltha-lib-go/pkg/kafka/client.go
2) An embedded Etcd server that runs in-process and represents an
Etcd server.

Change-Id: I52a36132568e08c596bb4136918bebcb654a3b99
diff --git a/vendor/github.com/grpc-ecosystem/go-grpc-prometheus/server.go b/vendor/github.com/grpc-ecosystem/go-grpc-prometheus/server.go
new file mode 100644
index 0000000..322f990
--- /dev/null
+++ b/vendor/github.com/grpc-ecosystem/go-grpc-prometheus/server.go
@@ -0,0 +1,48 @@
+// Copyright 2016 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+// gRPC Prometheus monitoring interceptors for server-side gRPC.
+
+package grpc_prometheus
+
+import (
+	prom "github.com/prometheus/client_golang/prometheus"
+	"google.golang.org/grpc"
+)
+
+var (
+	// DefaultServerMetrics is the default instance of ServerMetrics. It is
+	// intended to be used in conjunction the default Prometheus metrics
+	// registry.
+	DefaultServerMetrics = NewServerMetrics()
+
+	// UnaryServerInterceptor is a gRPC server-side interceptor that provides Prometheus monitoring for Unary RPCs.
+	UnaryServerInterceptor = DefaultServerMetrics.UnaryServerInterceptor()
+
+	// StreamServerInterceptor is a gRPC server-side interceptor that provides Prometheus monitoring for Streaming RPCs.
+	StreamServerInterceptor = DefaultServerMetrics.StreamServerInterceptor()
+)
+
+func init() {
+	prom.MustRegister(DefaultServerMetrics.serverStartedCounter)
+	prom.MustRegister(DefaultServerMetrics.serverHandledCounter)
+	prom.MustRegister(DefaultServerMetrics.serverStreamMsgReceived)
+	prom.MustRegister(DefaultServerMetrics.serverStreamMsgSent)
+}
+
+// Register takes a gRPC server and pre-initializes all counters to 0. This
+// allows for easier monitoring in Prometheus (no missing metrics), and should
+// be called *after* all services have been registered with the server. This
+// function acts on the DefaultServerMetrics variable.
+func Register(server *grpc.Server) {
+	DefaultServerMetrics.InitializeMetrics(server)
+}
+
+// EnableHandlingTimeHistogram turns on recording of handling time
+// of RPCs. Histogram metrics can be very expensive for Prometheus
+// to retain and query. This function acts on the DefaultServerMetrics
+// variable and the default Prometheus metrics registry.
+func EnableHandlingTimeHistogram(opts ...HistogramOption) {
+	DefaultServerMetrics.EnableHandlingTimeHistogram(opts...)
+	prom.Register(DefaultServerMetrics.serverHandledHistogram)
+}