Fix: Use dashboard time range in prometheus variable editor

[itsmylife]

What is this feature?

While using a non-default time range on a Prometheus dashboard, variable editor queries always end up being queries with the default time range. This was happening because 'languageProviderhad never received the time range. This PR makes surelanguageProvider` receives the right time range always.

Why do we need this feature?

A better variable support in Prometheus

Who is this feature for?

Prometheus users

Which issue(s) does this PR fix?:

Fixes #78947

Special notes for your reviewer:

How to test

• Create a Prometheus dashboard
• Set the time range as a non-default value like the last 2 days
• Go to settings and add a new variable
• Select label values type and select the metric
• In the network tab observe that the label query payload
  • end - start is 6h which is the default time range
• Switch to this branch and try again
• Observe that the queries have the right time range

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[bohandley]

LGTM! 🚀

[grafana-delivery-bot]

The backport to v10.2.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-78950-to-v10.2.x origin/v10.2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 46044efdf8592f88e59a6cc277a98370a446e9b7

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-78950-to-v10.2.x
# Create the PR body template
PR_BODY=$(gh pr view 78950 --json body --template 'Backport 46044efdf8592f88e59a6cc277a98370a446e9b7 from #78950{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "[v10.2.x] Fix: Use dashboard time range in prometheus variable editor" --body-file - --label "type/bug" --label "datasource/Prometheus" --label "area/frontend" --label "add to changelog" --label "no-changelog" --label "backport" --base v10.2.x --milestone 10.2.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-78950-to-v10.2.x

# Create a pull request where the `base` branch is `v10.2.x` and the `compare`/`head` branch is `backport-78950-to-v10.2.x`.

# Remove the local backport branch
git switch main
git branch -D backport-78950-to-v10.2.x