flaky TestConcurrentFetchers/fetch_records_produced_after_startup

[seizethedave]

--- FAIL: TestConcurrentFetchers (1.12s)
    --- FAIL: TestConcurrentFetchers/fetch_records_produced_after_startup (1.02s)
        logging.go:33: level info component kafka_client msg producing to a new topic for the first time, fetching metadata to learn its partitions topic test-topic
        logging.go:33: level info component kafka_client msg immediate metadata update triggered why forced load because we are producing to a topic for the first time
        logging.go:33: level info component kafka_client msg initializing producer id
        logging.go:33: level info component kafka_client msg done waiting for metadata for new topic topic test-topic
        logging.go:33: level info component kafka_client msg producer id initialization success id 6326[88](https://github.com/grafana/mimir/actions/runs/11946736248/job/33301576622#step:8:89)2942567818996 epoch 0
        fetcher_test.go:379: 
            	Error Trace:	/__w/mimir/mimir/pkg/storage/ingest/fetcher_test.go:379
            	Error:      	Not equal: 
            	            	expected: 2
            	            	actual  : 3
            	Test:       	TestConcurrentFetchers/fetch_records_produced_after_startup
        fetcher_test.go:1451: 
            	Error Trace:	/__w/mimir/mimir/pkg/storage/ingest/fetcher_test.go:1451
            	            				/__w/mimir/mimir/pkg/storage/ingest/fetcher_test.go:382
            	Error:      	Should be zero, but was 1
            	Test:       	TestConcurrentFetchers/fetch_records_produced_after_startup
            	Messages:   	expected there aren't any buffered records
        fetcher_test.go:1453: kgo.Fetches{}
FAIL
FAIL	github.com/grafana/mimir/pkg/storage/ingest	[97]

(In GitHub CI: https://github.com/grafana/mimir/actions/runs/11946736248/job/33301576622)