fix: make hostmetrics dashboard single-runner drilldown

[cbartz]

What this PR does

Reframes the runner VM hostmetrics Grafana dashboard from a multi-runner overview into a single-runner drilldown.

• The github_runner template variable now requires a single selection (includeAll: false, allValue removed).
• All 28 PromQL queries change github_runner=~"$github_runner" → github_runner="$github_runner" so panels chart one host instead of overlaying every match.
• Repo/workflow/job filters stay regex-matched and still default to "All" — they now act as a way to narrow the runner picker.
• Dashboard description updated to reflect the new behavior.

Why we need it

With more than a handful of active runners, the dashboard overlaid every series in scope on each panel, making it unreadable by default. Forcing a single-runner selection makes the panels useful out of the box; users still reach the runner they care about via the repo/workflow/job filters.

Screenshot

Test plan

• Loaded the modified dashboard JSON in Grafana against the COS Prometheus datasource.
• Verified the runner dropdown no longer offers "All" and forces a single value.
• Confirmed each panel renders one series per device/mountpoint/state for the selected runner.

Review focus

• The github_runner label match changed from =~ to =. Emitted values are GitHub runner hostnames, so regex-metachar collisions are not expected, but worth a sanity check.
• Dashboard description copy.

Checklist

• Changes comply with the project's coding standards and guidelines (see CONTRIBUTING.md and STYLE.md)
• CONTRIBUTING.md has been updated upon changes to the contribution/development process (e.g. changes to the way tests are run) — n/a
• Technical author has been assigned to review the PR in case of documentation changes (usually *.md files) — n/a, only dashboard JSON changed
• I updated docs/changelog.md with user-relevant changes — n/a, changelog scope is the Planner and Webhook gateway charms
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration) — n/a, dashboard JSON is not covered by tests
• If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct) — n/a
• If this PR involves a Grafana dashboard: I added a screenshot of the dashboard — pending: drop /tmp/pic.png into the Screenshot section above before marking ready for review
• If this PR involves Terraform: terraform fmt passes and tflint reports no errors — n/a
• If this PR involves Rockcraft: I updated the version — n/a

[Copilot]

Pull request overview

Updates the runner VM hostmetrics Grafana dashboard to behave as a single-runner drilldown rather than a multi-runner overlay, improving readability and making the default view actionable.

Changes:

• Forces github_runner to be single-select by disabling the “All” option.
• Updates PromQL queries to use exact label matching for github_runner (=) instead of regex matching (=~).
• Revises the dashboard description to reflect the new single-runner drilldown workflow.