gnmi-gateway behavior confirmation

[chiache11]

I tried to subscribe some path from router with STREAM/SAMPLE mode and use Kafka as exporter.
My target file is below.

{
  "request": {
    "default": {
      "subscribe": {
        "prefix": {
        },
        "subscription": [
          {
            "path": {
              "elem": [
                {
                  "name": "junos"
                },
                {
                  "name": "system"
                },
                {
                  "name": "linecard"
                },
                {
                  "name": "firewall"
                }
              ]
            },
                "mode":"SAMPLE",
                "sample_interval":2000000000
          }
        ],
        "encoding":"PROTO"
      }
    }
  },
  "target": {
    "MX204": {
      "addresses": [
        "192.168.22.22:32767"
      ],
      "credentials": {
        "username": "test",
        "password": "password"
      },
      "request": "default",
      "meta":{"NoTLSVerify":"yes"}
    }
  }
}

I used the command below to run gnmi-gateway process with parameters.

[root@server gnmi-gateway]$ ./gnmi-gateway -Exporters=kafka,debug -TargetLoaders=json -TargetJSONFile=targets.json -ExporterKafkaBrokers 192.168.56.136:9090 -ExporterKafkaTopic telemetry

And I also show the subscribe info in the router.

lab@mx204> show network-agent statistics gnmi

Subscription Details :
    Subscription ID                     : 19
    Type                                : gnmi
    Client IP                           : ipv6:::ffff:192.168.22.134:432
    GNMI mode                           : STREAM

    Sensor Statistics :
        Sensor Path                     : /junos/system/linecard/firewall/
        Reporting Interval              : 2
        Component(s)                    : PFE
        Component ID                    : 0
        Sub Component ID                : 0
        GNMI Sub Mode                   : SAMPLE

According to the information above.
I expected the router would send the data every 2 seconds, and gnmi-gateway would periodically send the data to kafka(or stdout).
However I saw the update data only send when the value changed.
Is this behavior same as expected or I need to modify some parameter to match the expected behavior?