Data race in `dskit/ring`

Discovered in [CI tests](https://github.com/grafana/pyroscope/actions/runs/13156843874/job/36715836676?pr=3892) in `pkg/test/integration`.

This has never been observed in the wild (we do not build with `race`, though). I'm not sure if the race is present in `dskit/ring` or in our tests.

```
==================
WARNING: DATA RACE
Write at 0x00c00d841860 by goroutine 40807:
runtime.mapassign_faststr()
/opt/hostedtoolcache/go/1.22.11/x64/src/runtime/map_faststr.go:203 +0x0
github.com/grafana/dskit/ring.(*Desc).mergeWithTime()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/ring/model.go:222 +0xdda
github.com/grafana/dskit/ring.(*Desc).Merge()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/ring/model.go:188 +0x5c
github.com/grafana/dskit/kv/memberlist.computeNewValue()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:1426 +0x58
github.com/grafana/dskit/kv/memberlist.(*KV).mergeValueForKey()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:1377 +0x2d3
github.com/grafana/dskit/kv/memberlist.(*KV).mergeBytesValueForKey()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:1358 +0x1ad
github.com/grafana/dskit/kv/memberlist.(*KV).MergeRemoteState()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:1319 +0x484
github.com/hashicorp/memberlist.(*Memberlist).mergeRemoteState()
/home/runner/go/pkg/mod/github.com/grafana/memberlist@v0.3.1-0.20220708130638-bd88e10a3d91/net.go:1261 +0x25b
github.com/hashicorp/memberlist.(*Memberlist).pushPullNode()
/home/runner/go/pkg/mod/github.com/grafana/memberlist@v0.3.1-0.20220708130638-bd88e10a3d91/state.go:664 +0x174
github.com/hashicorp/memberlist.(*Memberlist).Join()
/home/runner/go/pkg/mod/github.com/grafana/memberlist@v0.3.1-0.20220708130638-bd88e10a3d91/memberlist.go:275 +0x4da
github.com/grafana/dskit/kv/memberlist.(*KV).joinMembersBatch()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:678 +0xc4
github.com/grafana/dskit/kv/memberlist.(*KV).joinMembersInBatches()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:655 +0x127
github.com/grafana/dskit/kv/memberlist.(*KV).joinMembersWithRetries()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:600 +0x2c4
github.com/grafana/dskit/kv/memberlist.(*KV).joinMembersOnStartup()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:575 +0x44b
github.com/grafana/dskit/kv/memberlist.(*KV).running()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/kv/memberlist/memberlist_client.go:476 +0x6a
github.com/grafana/dskit/kv/memberlist.(*KV).running-fm()
:1 +0x47
github.com/grafana/dskit/services.(*BasicService).main()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/services/basic_service.go:190 +0x3b7
github.com/grafana/dskit/services.(*BasicService).StartAsync.func1.gowrap1()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/services/basic_service.go:119 +0x33

Previous read at 0x00c00d841860 by goroutine 40943:
runtime.mapiterinit()
/opt/hostedtoolcache/go/1.22.11/x64/src/runtime/map.go:816 +0x0
github.com/grafana/dskit/ring.(*Lifecycler).updateCounters()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/ring/lifecycler.go:920 +0x126
github.com/grafana/dskit/ring.(*Lifecycler).initRing()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/ring/lifecycler.go:712 +0x7fc
github.com/grafana/dskit/ring.(*Lifecycler).loop()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/ring/lifecycler.go:465 +0x68
github.com/grafana/dskit/ring.(*Lifecycler).loop-fm()
:1 +0x47
github.com/grafana/dskit/services.(*BasicService).main()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/services/basic_service.go:190 +0x3b7
github.com/grafana/dskit/services.(*BasicService).StartAsync.func1.gowrap1()
/home/runner/go/pkg/mod/github.com/grafana/dskit@v0.0.0-20231221015914-de83901bf4d6/services/basic_service.go:119 +0x33

```