feat: improve placeholder replacement of byte sizes

[na--]

What this PR does / why we need it:

It improves the placeholder replacement for byte sizes with spaces in the middle (e.g. 3.14 GiB, 2 KiB, etc.) and in plain bytes (e.g. 3b, 1337 B, etc.).

This was especially problematic for some lines like the distributor's push request parsed messages, see example in the tests.

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• 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.