[dataset] Add dataset "system uptime" into non-db client. (#52)

Motivation of this PR:
This PR aims to enable the streaming telemetry container to stream out the system uptime of SONiC Host. This dataset is added into non-database client since it is a kind of value which will be refreshed periodically.

How can I do that?
I follow the example of dataset in non-database client such as meminfo to do the implementation. The data source of system uptime is from the file /proc/uptime on the SONiC host.

We can use the command ./gnmi_cli -client_types=gnmi -a <DuT_IP>:8080 -t OTHERS -logtostderr -insecure -qt p -pi 10s -q proc/uptime to query the system uptime every 10 seconds.

admin@str-a7050-acs-3:~$ ./gnmi_cli -client_types=gnmi -a localhost:8080 -t OTHERS -logtostderr -insecure -qt p -pi 10s -q proc/uptime
{
"OTHERS": {
"proc": {
"uptime": "{"total":314.74,"idle":981.27}"
}
}
}
{
"OTHERS": {
"proc": {
"uptime": "{"total":324.74,"idle":1016.85}"
}
}
}

sonic_data_client/non_db_client.go

@@ -73,6 +73,10 @@ var (
 			path:    []string{"OTHERS", "platform", "cpu"},
 			getFunc: dataGetFunc(getCpuUtil),
 		},
+		{ // Get host uptime
+			path:    []string{"OTHERS", "proc", "uptime"},
+			getFunc: dataGetFunc(getSysUptime),
+		},
 		{ // Get proc meminfo
 			path:    []string{"OTHERS", "proc", "meminfo"},
 			getFunc: dataGetFunc(getProcMeminfo),
@@ -280,6 +284,18 @@ func getProcStat() ([]byte, error) {
 	return b, nil
 }
 
+func getSysUptime() ([]byte, error) {
+    uptime, _ := linuxproc.ReadUptime("/proc/uptime")
+    b, err := json.Marshal(uptime)
+    if err != nil {
+        log.V(2).Infof("%v", err)
+        return b, err
+    }
+
+    log.V(4).Infof("getSysUptime, output %v", string(b))
+    return b, nil
+}
+
 func getBuildVersion() ([]byte, error) {
 
 	// Load and parse the content of version file