fix: offsetFetchResponse missing topic partitions

[sciclon2]

Summary

Following the upgrade of the kafka_Exporter, a discrepancy in metrics was discovered, specifically involving consumer groups in an "Empty" state not being tracked. Upon examination, it was observed that the offsetFetchResponse did not contain necessary information for the consumer group identified as "pipes-pro-1dfe17f5-c4d9-4be2-bf46-144f03a60a36".

Worth to mention this CG is committing offsets and working properly, the client is manually handling it, that is the reason it shows up in Empty state

To investigate further, the exporter was initiated with additional logging parameters to capture details about the offsetFetchResponse. Running the exporter command showed that the initial response had a Version 1 with empty mapping: offsetFetchResponse: &{1 0 map[] .

Upon switching to Version 4, the mapping included the requisite information, allowing for successful tracking of consumer group activities: offsetFetchResponse: &{4 0 map[Private-Pipes-Synthetic-10-Events-Partition-Every-Second:map[0:0x40065f7b00 1:0x40065f73b0 2:0x40065f7e00 3:0x40065f7aa0 4:0x40065f7770 5:0x40065f7e60 6:0x40065f7dd0 7:0x40065f77a0 8:0x40065f7620 9:0x40065f7e90 10:0x40057f2000 11:0x40065f7da0 12:0x40065f7b60 13:0x40065f7560 14:0x40057f2030 15:0x40065f7590 16:0x40065f7b90 17:0x40065f7c50 18:0x40065f7cb0 19:0x40065f7bf0 20:0x40065f73e0 21:0x40065f75c0 22:0x40065f7ce0 23:0x40065f7bc0]].

Moreover, it was noted that the fetchOffsetVersion() function, previously utilized for this purpose, had been commented out, leading to the Version being hardcoded within the OffsetFetchRequest function.

[danielqsj]

LGTM, thanks @sciclon2