VOL-4788 : Lib in in voltha-libs-go for support of prometheus counters in voltha

Change-Id: I1a4a81f775595b89dbc2a5e4411e84034e30e1af
diff --git a/pkg/stats/manager.go b/pkg/stats/manager.go
new file mode 100644
index 0000000..fdc8cc0
--- /dev/null
+++ b/pkg/stats/manager.go
@@ -0,0 +1,76 @@
+/*
+ * Copyright 2018-present Open Networking Foundation
+
+ * 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 stats
+
+import (
+	"context"
+	"time"
+)
+
+type CollectorName string
+
+const (
+	OltAdapter CollectorName = "adapter_olt"
+	OnuAdapter CollectorName = "adapter_onu"
+	VCore      CollectorName = "rw_core"
+)
+
+func (s CollectorName) String() string {
+	switch s {
+	case OltAdapter:
+		return "adapter_olt"
+	case OnuAdapter:
+		return "adapter_onu"
+	case VCore:
+		return "rw_core"
+	}
+	return "unknown"
+}
+
+type StatsManager interface {
+	// Start starts the statistics manager with name and makes the collected stats available at port p.
+	Start(ctx context.Context, p int, name CollectorName)
+
+	//CountForDevice counts the number of times the counterName happens for device devId with serial number sn. Each call to Count increments it by one.
+	CountForDevice(devId, sn string, counterName DeviceCounter)
+
+	//AddForDevice adds val to counter.
+	AddForDevice(devId, sn string, counter DeviceCounter, val float64)
+
+	//CollectDurationForDevice calculates the duration from startTime to time.Now() for device devID with serial number sn.
+	CollectDurationForDevice(devID, sn string, dName NonDeviceDuration, startTime time.Time)
+
+	//Count counts the number of times the counterName happens. Each call to Count increments it by one.
+	Count(counter NonDeviceCounter)
+
+	//Add adds val to counter.
+	Add(counter NonDeviceCounter, val float64)
+
+	//CollectDuration calculates the duration from startTime to time.Now().
+	CollectDuration(dName NonDeviceDuration, startTime time.Time)
+}
+
+type NullStatsServer struct {
+}
+
+func (n *NullStatsServer) Start(ctx context.Context, p int, name CollectorName)              {}
+func (n *NullStatsServer) CountForDevice(devId, sn string, counterName DeviceCounter)        {}
+func (n *NullStatsServer) AddForDevice(devId, sn string, counter DeviceCounter, val float64) {}
+func (n *NullStatsServer) CollectDurationForDevice(devID, sn string, dName NonDeviceDuration, startTime time.Time) {
+}
+func (n *NullStatsServer) Count(counter NonDeviceCounter)                               {}
+func (n *NullStatsServer) Add(counter NonDeviceCounter, val float64)                    {}
+func (n *NullStatsServer) CollectDuration(dName NonDeviceDuration, startTime time.Time) {}