fix: cinder_limits_volume_used_gb metric

[Tadas Sutkaitis (fitbeard)]

Fixes: #92

Limits → read hard_limit from quotas (which only exists when explicitly set)
Usage → read in_use directly from quota_usages (which always exists when resources are consumed)

[Copilot]

Pull request overview

Fixes incorrect Cinder limits_volume_used_gb reporting when a project has usage records but no explicit quota row by separating quota limits and quota usage data sources.

Changes:

• Update Cinder quota limits query to read only hard_limit from quotas.
• Add a new query to read in_use directly from quota_usages and overlay it in the limits collector.
• Extend unit + integration tests to cover “usage without explicit quota” (issue #92).

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sql/cinder/queries.sql	Splits limits vs usage into distinct SQL queries (GetProjectQuotaLimits, GetProjectQuotaUsages).
internal/db/cinder/queries.sql.go	Regenerates sqlc output for the updated/added queries and result structs.
internal/collector/cinder/limits.go	Collects limits from quotas and usage from quota_usages, overlaying usage per project.
internal/collector/cinder/limits_test.go	Updates mocks/expected output to reflect the new query split; adds regression test for issue #92.
internal/collector/cinder/integration_test.go	Adds integration coverage for usage existing without quota rows.
.gitignore	Ignores exporter.sh.

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