Add jsz consumer ack floor consumer_seq and stream_seq metrics

[phho]

The latest version nats exporter doesn't contain ack floor metrics, but the stream/consumer ack floor seq can be obtained through nats-server monitoring endpoint /jsz. Please consider to add ack floor metrics in nats exporter for monitoring delivered seq and ack floor seq difference. Thanks.

/jsz?consumers=true output as below

"account_details": [
 ...
      "stream_detail": [
        {
          "name": "my-stream",
           ...
          "consumer_detail": [
            {
              "stream_name": "my-stream",
              "name": "my-consumer",
               ...
              "ack_floor": {
                "consumer_seq": 5,
                "stream_seq": 5
              },