chore: rename consumption lag variable

[grobinson-grafana]

What this PR does / why we need it:

This renames the variable for two reasons:

1. It matches the metric name
2. I want to add a second metric that tracks consumption lag offset, not just seconds

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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