native histogram: Fix race between Write and addExemplar

[krajorama]

Follow-up to #1608

Fixes

2024-09-06T08:32:44.9910707Z 08:29:59 ingester-3: WARNING: DATA RACE
2024-09-06T08:32:44.9911367Z 08:29:59 ingester-3: Read at 0x00c001b9f4d8 by goroutine 480:
2024-09-06T08:32:44.9912309Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*histogram).Write()
2024-09-06T08:32:44.9913404Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/histogram.go:849 +0x16ef
2024-09-06T08:32:44.9914502Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*wrappingMetric).Write()
2024-09-06T08:32:44.9915647Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/wrap.go:172 +0x65
2024-09-06T08:32:44.9916698Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*wrappingMetric).Write()
2024-09-06T08:32:44.9917746Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/wrap.go:172 +0x65
2024-09-06T08:32:44.9919099Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*wrappingMetric).Write()
2024-09-06T08:32:44.9920110Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/wrap.go:172 +0x65
2024-09-06T08:32:44.9921088Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.processMetric()
2024-09-06T08:32:44.9922271Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/registry.go:633 +0x112
2024-09-06T08:32:44.9923296Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*Registry).Gather()
2024-09-06T08:32:44.9924408Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/registry.go:502 +0xe8a
2024-09-06T08:32:44.9925524Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*noTransactionGatherer).Gather()
2024-09-06T08:32:44.9926636Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/registry.go:1074 +0x3a
2024-09-06T08:32:44.9927818Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus/promhttp.HandlerForTransactional.func1()
2024-09-06T08:32:44.9929026Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/promhttp/http.go:165 +0x3fa

vs

2024-09-06T08:32:44.9959927Z 08:29:59 ingester-3: Previous write at 0x00c001b9f4d8 by goroutine 370:
2024-09-06T08:32:44.9960836Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*nativeExemplars).addExemplar()
2024-09-06T08:32:44.9962035Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/histogram.go:1706 +0x188b
2024-09-06T08:32:44.9963229Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*histogram).updateExemplar()
2024-09-06T08:32:44.9964331Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/histogram.go:1140 +0x116
2024-09-06T08:32:44.9965403Z 08:29:59 ingester-3: github.com/prometheus/client_golang/prometheus.(*histogram).ObserveWithExemplar()
2024-09-06T08:32:44.9966601Z 08:29:59 ingester-3: /__w/mimir/mimir/vendor/github.com/prometheus/client_golang/prometheus/histogram.go:770 +0x69
2024-09-06T08:32:44.9967591Z 08:29:59 ingester-3: github.com/grafana/dskit/instrument.ObserveWithExemplar()

[beorn7]

Nice, thank you.

(And as usual, handing over to @bwplotka @ArthurSens @kakkoyun for final check and merge.)