add concurrent workers for consumer group metrics collection

[donovanbai-dd]

This PR speeds up consumer group metric collection, which can be very slow for large kafka clusters.

changes:

1. Add --group.workers (default is 100) and --group.metrics.timeout (default is 5m). Instead of collecting metrics for each consumer group serially for each broker, use a separate goroutine for each group.
2. For each consumer group, make a copy of the broker object before calling FetchOffset since each broker object does not support concurrent requests.
3. Remove an unnecessary mutex lock that was happening for each partition of each topic.
4. Add some debug logging for execution time details

Results on a test cluster
Before: 2 minutes to collect consumer group metrics
After: 14 seconds to collect consumer group metrics

[donovanbai-dd]

I had to change this due to https://stackoverflow.com/questions/78519711/toolchain-not-available-error-prevents-me-from-using-any-go-commands

[danielqsj]

lgtm, please fix the golang lint issue @donovanbai-dd https://github.com/danielqsj/kafka_exporter/actions/runs/20580739843/job/59777223997?pr=506

[donovanbai-dd]

lgtm, please fix the golang lint issue @donovanbai-dd https://github.com/danielqsj/kafka_exporter/actions/runs/20580739843/job/59777223997?pr=506

it's fixed now