[buddy] Discovery service panics on GKE clusters without labels

[tigrato]

Fixes #30619.

For a GKE cluster without labels like

$ gcloud container clusters list --format='json(status,resourceLabels)'
[
  {
    "status": "RUNNING"
  }
]

, Teleport with discovery_service enabled for GKE

discovery_service:
  enabled: "yes"
  discovery_group: "main"
  gcp:
    - types: ["gke"]
      locations: ["*"]
      project_ids: ["some-project-id-123456789"]
      tags:
        "*" : "*"

panics with the following output in its log

panic: assignment to entry in nil map
goroutine 363 [running]:
github.com/gravitational/teleport/lib/services.labelsFromGCPKubeCluster(...)
        github.com/gravitational/teleport/lib/services/kubernetes.go:260
github.com/gravitational/teleport/lib/services.NewKubeClusterFromGCPGKE({{0xc0024572b8, 0x4}, {0x0, 0x0}, {0xc002457560, 0xc}, {0xc00159a198, 0x17}, 0x2, 0x0})
        github.com/gravitational/teleport/lib/services/kubernetes.go:235 +0x1dc
github.com/gravitational/teleport/lib/srv/discovery/fetchers.(*gkeFetcher).getMatchingKubeCluster(0xc001580190, {{0xc0024572b8, 0x4}, {0x0, 0x0}, {0xc002457560, 0xc}, {0xc00159a198, 0x17}, 0x2, ...})
        github.com/gravitational/teleport/lib/srv/discovery/fetchers/gke.go:137 +0x58
github.com/gravitational/teleport/lib/srv/discovery/fetchers.(*gkeFetcher).getGKEClusters(0xc001580190, {0x98466c0?, 0xc0015806e0?})
        github.com/gravitational/teleport/lib/srv/discovery/fetchers/gke.go:96 +0x145
github.com/gravitational/teleport/lib/srv/discovery/fetchers.(*gkeFetcher).Get(0x0?, {0x98466c0?, 0xc0015806e0?})
        github.com/gravitational/teleport/lib/srv/discovery/fetchers/gke.go:82 +0x3b
github.com/gravitational/teleport/lib/srv/discovery/common.(*Watcher).fetchAndSend.func1()
        github.com/gravitational/teleport/lib/srv/discovery/common/watcher.go:129 +0x70
golang.org/x/sync/errgroup.(*Group).Go.func1()
        golang.org/x/sync@v0.3.0/errgroup/errgroup.go:75 +0x64
created by golang.org/x/sync/errgroup.(*Group).Go
        golang.org/x/sync@v0.3.0/errgroup/errgroup.go:72 +0xa5

The reason for that is because

1. the GCP client sets Labels equal to nil for a GKE cluster without labels.
2. the Clone() function in exp/maps returns nil when a nil is
   passed as an argument; see [0].

Fixed that by creating an empty labels map and then populating it using Copy().

Co-authored-by: @tigrato

[0] https://go-review.googlesource.com/c/exp/+/417274

buddies: #30618

[public-teleport-github-review-bot]

@tigrato See the table below for backport results.

Branch	Result
branch/v11	Failed
branch/v12	Failed
branch/v13	Failed