Not able to get lag or status info from endpoint on existing consumers

Hi, I'm trying to monitor Kafka consumers on our staging machine, below is the config file:

`
[general]
pidfile="./burrow.pid"
stdout-logfile="burrow.out"

[logging]
filename="log/burrow.log"
level="debug"
maxsize=100
maxbackups=30
maxage=10
use-localtime=false
use-compression=true

[zookeeper]
servers=[ "example.com:2181" ]
timeout=6

[client-profile.stage]
kafka-version="0.10.2"

[client-profile.production]
kafka-version="0.10.2"

[cluster.stage]
class-name="kafka"
servers=[ "example.com:9092" ]
client-profile="stage"
topic-refresh=120
offset-refresh=30
zookeeper-offset=true

[consumer.stage]
class-name="kafka"
cluster="stage"
servers=[ "example.com:9092" ]
client-profile="stage"
offsets-topic="__consumer_offsets"
#group-blacklist="^(console-consumer-|python-kafka-consumer-|quick-).*$"
group-blacklist=".*"
group-whitelist=""

[httpserver.default]
address=":18002"
`

the thing is I am able to see all the consumers using the corresponding endpoint but can't get the lag on the existing consumers, below is a snippet of the log:
`
{"level":"debug","ts":1524773696.0555763,"msg":"dropped","type":"module","coordinator":"storage","class":"inmemory","name":"default","worker":7,"cluster":"stage","consumer":"ca_raw","topic":"ca_raw","partition":1,"topic_partition_count":0,"offset":24,"timestamp":1484519101275,"owner":"","request":"StorageSetConsumerOffset","reason":"old offset"}`

also this is the result I got from hitting the /lag endpoint:

```
{
"error":
false,
"message":
"consumer status returned",
"request":
{
"host":
"MacBook-Pro.local",
"url":
"/v3/kafka/stage/consumer/ca_raw/lag"
},
"status":
{
"cluster":
"stage",
"complete":
1,
"group":
"ca_raw",
"maxlag":
null,
"partition_count":
0,
"partitions":
[],
"status":
"NOTFOUND",
"totallag":
0
}
}
```

Let me know if you need more information, thx in advance!

Start with the `/v3/kafka/stage/consumer/ca_raw/lag` endpoint and the supplied Burrow configuration, then trace the `StorageSetConsumerOffset` log entry showing `topic_partition_count: 0` and `reason: "old offset"`. Reproduce the request against the existing consumer and determine why it returns `NOTFOUND`; done means the endpoint reports the consumer's lag or the missing prerequisite is clearly identified.

Written by the indexing model from the issue text.