add missing metadata and assignment methods in describegroups

[pcallewaert]

The code did already exists in sync_group_response.go, but added them also in describe_groups_response.go.
Now you can read the MemberAssignment and MemberMetadata also (before it was raw bytes, and not that easy to get the actual structs)

[wvanbergen]

Can you add a unit test for this?

[eapache]

Also please be consistent with the variable name you assign the GroupMemberDescription; the rest of the file uses gmd.

[pcallewaert]

@eapache my bad, fixed

@wvanbergen I started adding them, but the encoder/decoder is already tested in consumer_group_members_test.go. This is also being used in the SyncGroupResponse. In SyncGroupResponse only the bytes are tested, so following this practice I did not add them.

[pcallewaert]

Any update on this? :)

[eapache]

Sorry about that. Looks fine to me. @wvanbergen ?

[wvanbergen]

👍 - I'd still like to see some tests for this but if we're currently not doing that elsewhere either, I am OK with leaving that for now.