chore: Call shardstreams.Config by value instead of by reference

[DylanGuedes]

What this PR does / why we need it:
Use shardstreams.Config by value instead of by reference to fix docs generation.
Our docs-generator tool relies on the struct address/references to assume that flags are present. Using this config by value fixes it.

Which issue(s) this PR fixes:
Fixes #12904

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

[grafanabot]

This PR must be merged before a backport PR will be created.

[MichelHollands]

LGTM

[grafanabot]

The backport to release-3.0.x 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-12915-to-release-3.0.x origin/release-3.0.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x afd9e363065ee2bb07844f8dbb30cc8dfb310c9d

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-12915-to-release-3.0.x
# Create the PR body template
PR_BODY=$(gh pr view 12915 --json body --template 'Backport afd9e363065ee2bb07844f8dbb30cc8dfb310c9d from #12915{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "chore: [release-3.0.x] chore: Call `shardstreams.Config` by value instead of by reference" --body-file - --label "size/M" --label "type/docs" --label "backport" --base release-3.0.x --milestone release-3.0.x --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-12915-to-release-3.0.x

# Create a pull request where the `base` branch is `release-3.0.x` and the `compare`/`head` branch is `backport-12915-to-release-3.0.x`.

# Remove the local backport branch
git switch main
git branch -D backport-12915-to-release-3.0.x