Store: Fix data race in advertised label set in bucket store

[matej-g]

Signed-off-by: Matej Gera matejgera@gmail.com

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

As a part of #5066, we have detected some data races. One of them is occurring due to concurrent write / read of bucket store's label set, excerpt:

12:05:39 store-gw-1: ==================
12:05:39 store-gw-1: WARNING: DATA RACE
12:05:39 store-gw-1: Read at 0x00c0001c2450 by goroutine 36:
12:05:39 store-gw-1: github.com/thanos-io/thanos/pkg/store.(*BucketStore).LabelSet()
12:05:39 store-gw-1: /go/src/github.com/thanos-io/thanos/pkg/store/bucket.go:687 +0x3d
12:05:39 store-gw-1: main.runStore.func6()
12:05:39 store-gw-1: /go/src/github.com/thanos-io/thanos/cmd/thanos/store.go:393 +0x1d8
12:05:39 store-gw-1: github.com/thanos-io/thanos/pkg/info.(*InfoServer).Info()
...
12:05:39 store-gw-1: Previous write at 0x00c0001c2450 by goroutine 110:
12:05:39 store-gw-1: github.com/thanos-io/thanos/pkg/store.(*BucketStore).SyncBlocks()
12:05:39 store-gw-1: /go/src/github.com/thanos-io/thanos/pkg/store/bucket.go:511 +0xcc4
...

In addition, I simplified the Info server method defaults.

Verification

Ran tests locally with #5066; data race not being detected anymore.