chore(mixin): Allow regex filter for log level label

[naing2victor]

What this PR does / why we need it:
Allow regex filter on log level label because variables selector dropdown has multi-select option enabled. Regex filter for log level label is also used in helm dashboard

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
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

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[trevorwhitney]

have you done any benchmarks here? regex will be slower so I'd like to make sure I fully understand the use case, mind providing a few screenshots and brief explanation of your flow?

[naing2victor]

@trevorwhitney I don't have specific use case for this. Since i found regex is used for log level label in helm dashboard and i thought if we want to make it standard across the repo. We can set multi-value selector disable, so that Log Rate and Logs panel will show proper result. Right now if user select multiple options for log level, above 2 panels will not show data.