FAB-12592 Create a stats handler for the server

Create counters for grpc connections. There will be
one counter that will track number of connections opened,
and the second counter will track the number of connections
closed.

Change-Id: I218886e4b7f3710bd342129526d5555c9fe9701c
Signed-off-by: Saad Karim <skarim@us.ibm.com>

core/comm/serverstatshandler.go

@@ -0,0 +1,38 @@
+/*
+Copyright IBM Corp. All Rights Reserved.
+
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package comm
+
+import (
+	"context"
+
+	"github.com/go-kit/kit/metrics"
+	"google.golang.org/grpc/stats"
+)
+
+type ServerStatsHandler struct {
+	OpenConnCounter   metrics.Counter
+	ClosedConnCounter metrics.Counter
+}
+
+func (h *ServerStatsHandler) TagRPC(ctx context.Context, info *stats.RPCTagInfo) context.Context {
+	return ctx
+}
+
+func (h *ServerStatsHandler) HandleRPC(ctx context.Context, s stats.RPCStats) {
+	switch s.(type) {
+	case *stats.Begin:
+		h.OpenConnCounter.Add(1)
+	case *stats.End:
+		h.ClosedConnCounter.Add(1)
+	}
+}
+
+func (h *ServerStatsHandler) TagConn(ctx context.Context, info *stats.ConnTagInfo) context.Context {
+	return ctx
+}
+
+func (h *ServerStatsHandler) HandleConn(ctx context.Context, s stats.ConnStats) {}

core/comm/serverstatshandler_test.go

@@ -0,0 +1,37 @@
+/*
+Copyright IBM Corp. All Rights Reserved.
+
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package comm_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/hyperledger/fabric/common/metrics/metricsfakes"
+	"github.com/hyperledger/fabric/core/comm"
+	"github.com/stretchr/testify/assert"
+	"google.golang.org/grpc/stats"
+)
+
+func TestConnectionCounters(t *testing.T) {
+	openConn := &metricsfakes.Counter{}
+	closedConn := &metricsfakes.Counter{}
+	sh := &comm.ServerStatsHandler{
+		OpenConnCounter:   openConn,
+		ClosedConnCounter: closedConn,
+	}
+
+	for i := 1; i <= 10; i++ {
+		sh.HandleRPC(context.Background(), &stats.Begin{})
+	}
+	assert.Equal(t, 10, openConn.AddCallCount())
+
+	for i := 1; i <= 5; i++ {
+		sh.HandleRPC(context.Background(), &stats.End{})
+	}
+	assert.Equal(t, 5, closedConn.AddCallCount())
+
+}