chore: apply series limit per variant for multi-variant queries

[trevorwhitney]

What this PR does / why we need it:

Since multi-variant queries will result in many more series than their traditional counterparts, we need to change how we enforce limits on them. This PR changes the series limiter to apply the limit per variant for multi-variant queries.

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

[trevorwhitney]

Converted to the a draft. After talking with the frontend folks, I realized the importance of returning the variants that aren't over the limit while throwing away the ones that are. I'm refactoring to incorporate that into this PR.

It's ready again 🚀

[trevorwhitney]

I don't like this, it's going to double the memory needed. I'm going to change this on Tuesday to reuse the slice in the response.