Graphite: Fix bug in wildcard queries to Graphite plugin

[mmandrus]

What is this feature?

In this recent PR, the Graphite datasource was refactored to support multiple queries. However, this introduced a bug in wildcard queries due to the fact that it was overwriting results belonging to the same frame instead of appending them. This PR fixes that.

Public dashboard panel with wildcard queries

Why do we need this feature?

In order to make public dashboards accurately reflect what is shown on regular dashboards. Note that this has no bearing on regular dashboards, which query graphite through the datasource proxy endpoint.

Which issue(s) does this PR fix?:

Fixes #60141

[grafanabot]

Hello @mmandrus!
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 none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[mmandrus]

@grafana/observability-metrics do you agree that this should be backported? This really only affects public dashboards, but it's still a bug caused by a recent change.

[gtk-grafana]

@mmandrus yeah it's a one line fix with the type/bug label, I'd say that makes it applicable for backport. Go for it!

[grafanabot]

The backport to v9.3.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-60549-to-v9.3.x origin/v9.3.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 09c759b36c520c69038dc21b7fdc504df058b0db
# Push it to GitHub
git push --set-upstream origin backport-60549-to-v9.3.x
git switch main
# Remove the local backport branch
git branch -D backport-60549-to-v9.3.x

Then, create a pull request where the base branch is v9.3.x and the compare/head branch is backport-60549-to-v9.3.x.