feat: add structured metadata to the promtail push API

[slim-bean]

What this PR does / why we need it:

Loki added support for structured metadata in the push API some time ago, and Promtail was also updated to support this natively, however it was missed to add support for propagating structured metadata when using the push API in promtail, this PR addresses this.

Which issue(s) this PR fixes:
Fixes #

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

[loki-gh-app]

The backport to k218 failed:

Validation Failed: "Could not resolve to a node with the global id of 'MDQ6VGVhbTM0MjIwMDE='."

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-14153-to-k218 origin/k218
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 66cffcb427bda28af6fbcfcf85a34771db3787bc

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-14153-to-k218
# Create the PR body template
PR_BODY=$(gh pr view 14153 --json body --template 'Backport 66cffcb427bda28af6fbcfcf85a34771db3787bc from #14153{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'chore: [k218] feat: add structured metadata to the promtail push API' --body-file - --label 'size/S' --label 'product-approved' --label 'backport' --base k218 --milestone k218 --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-14153-to-k218

# Create a pull request where the `base` branch is `k218` and the `compare`/`head` branch is `backport-14153-to-k218`.

# Remove the local backport branch
git switch main
git branch -D backport-14153-to-k218

[loki-gh-app]

The backport to k218 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-14153-to-k218 origin/k218
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 66cffcb427bda28af6fbcfcf85a34771db3787bc

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-14153-to-k218
# Create the PR body template
PR_BODY=$(gh pr view 14153 --json body --template 'Backport 66cffcb427bda28af6fbcfcf85a34771db3787bc from #14153{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'chore: [k218] feat: add structured metadata to the promtail push API' --body-file - --label 'size/S' --label 'product-approved' --label 'backport' --base k218 --milestone k218 --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-14153-to-k218

# Create a pull request where the `base` branch is `k218` and the `compare`/`head` branch is `backport-14153-to-k218`.

# Remove the local backport branch
git switch main
git branch -D backport-14153-to-k218