Connector lag monitoring support

Nobody has claimed this yet.

Hi,

I'm currently giving a try with Burrow and running into some problems.
I'm trying to monitor lag for connectors (HDFS, JDBC, etc) and when I'm starting burrow on my test environment I get the following debug trace :

```
{"level":"debug","ts":1554735418.3128974,"msg":"skipped metadata because of unknown protocolType","type":"module","coordinator":"consumer","class":"kafka","name":"consumer_kafka","offset_topic":"__consumer_offsets","offset_partition":3,"offset_offset":1117,"message_type":"metadata","group":"connector.dev._connect_topic-namexxxx-mz0zx7t8u7-cluster","protocol_type":"connect","generation":1,"protocol":"default","leader":"connect-1-d4fcb6c5-91af-4f6a-a735-ad82fd748c36","current_state_timestamp":1553520383214}
{"level":"debug","ts":1554735418.3129158,"msg":"skipped metadata because of unknown protocolType","type":"module","coordinator":"consumer","class":"kafka","name":"consumer_kafka","offset_topic":"__consumer_offsets","offset_partition":3,"offset_offset":1118,"message_type":"metadata","group":"connector.dev._connect_topic-namexx-sourcexxxx-mz0zx7t8u7-cluster","protocol_type":"connect","generation":2,"protocol":"default","leader":"connect-1-d4fcb6c5-91af-4f6a-a735-ad82fd748c36","current_state_timestamp":1553520384339}
```
So I guess `connect` is not a supported protocolType ?
I see that this code was added 3 months ago and apparently is the cause of this `pruning`.
https://github.com/linkedin/Burrow/blob/3cdc7e73b8388985d95ea7f05ef23c7bec3e9985/core/internal/consumer/kafka_client.go#L433-L437

Is there any plan to add this support in the future ? or this is more a PEBKAC ?

FYI : I also tested to remove this code and I got another error when extracting the group metadata. It fail with a `"failed to decode"` `reason:assigment` here :
https://github.com/linkedin/Burrow/blob/3cdc7e73b8388985d95ea7f05ef23c7bec3e9985/core/internal/consumer/kafka_client.go#L588-L591

And caused by an error in the member assignment extraction : https://github.com/linkedin/Burrow/blob/3cdc7e73b8388985d95ea7f05ef23c7bec3e9985/core/internal/consumer/kafka_client.go#L611-L614

But I'm not sure that those two errors are related.

Start in core/internal/consumer/kafka_client.go, especially the protocolType handling around lines 433-437 and group metadata decoding around lines 588-614. Reproduce the connector-group trace, then determine whether the connect protocol can be supported without the reported metadata and member-assignment decoding failures. Done means connector lag groups are handled without being pruned or failing to decode.

Written by the indexing model from the issue text.