Remove backend name from VBE.* metrics

[alopezz]

What does this PR do?

Renames VBE.* metrics to remove the backend name, which is arbitrary and should not be part of the metric name.

Motivation

When reloading a backend, the backend name portion of the varnish.VBE.* metrics changes (and includes a timestamp), which is a problem because metric names become unbounded.

Additional Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached

[github-actions[bot]]

Label changelog/Changed was added to this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the changelog/Fixed label.

[github-actions[bot]]

Label changelog/Changed was added to this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the changelog/Fixed label.

[github-actions[bot]]

Label changelog/Changed was added to this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the changelog/Fixed label.

[codecov[bot]]

Codecov Report

Merging #12488 (bfde5d8) into master (188f7ce) will increase coverage by 0.00%. Report is 3026 commits behind head on master. The diff coverage is 89.47%.

Flag	Coverage Δ
varnish	84.57% <88.23%> (+0.24%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

[FlorentClarret]

@alopezz @steveny91 is this ready for review?