fix(gateway client): exit with nil pointer dereference panic when using ring mode

[littlepangdi]

What this PR does / why we need it:
Fixes gateway client return with nil pointer panic when using ring mode for ruler service.
clients.dnsProvider will only be init when using simple mode here:

loki/pkg/indexgateway/client.go

Line 184 in 1834065

sgClient.dnsProvider = dnsProvider

Service like ruler using ring mode will run into panic nil pointer reference during restarting or exiting:

Which issue(s) this PR fixes:
Fix #13437

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[chaudum]

Thanks for fixing!

[grafanabot]

Hello @chaudum!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[grafanabot]

Hello @chaudum!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[grafanabot]

The backport to k210 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-13436-to-k210 origin/k210
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 304db100b382f0c1d1d9999dfe8ca77d1ac901c9

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-13436-to-k210
# Create the PR body template
PR_BODY=$(gh pr view 13436 --json body --template 'Backport 304db100b382f0c1d1d9999dfe8ca77d1ac901c9 from #13436{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'chore: [k210] fix(gateway client): exit with nil pointer dereference panic when using ring mode' --body-file - --label 'size/XS' --label 'type/bug' --label 'backport' --base k210 --milestone k210 --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-13436-to-k210

# Create a pull request where the `base` branch is `k210` and the `compare`/`head` branch is `backport-13436-to-k210`.

# Remove the local backport branch
git switch main
git branch -D backport-13436-to-k210