feat: Do not add empty blooms to offsets

[salvacorts]

What this PR does / why we need it:

Series w/o structured metadata are appended to blocks with empty blooms, even though empty blooms are small, this adds up among all the blocks. This also prevent us from experimenting with single-stream blocks since we'd end up with many tiny blocks with empty blooms.

This PR solves this by only appending series to the index section if the series yield empty blooms, but not populating the offsets for the series. Now if we want to try out single-stream blocks, we can:

1. Set a really small block size target, so adding a single populated bloom will exceed the max size
2. Series with empty blooms will be part of a blocks that spans multiple series but where only a single series actually has a bloom.

This is obviously not optimal, but will allow us to test out single-stream blocks with what we currently have.

Besides allowing us to test single-stream blocks, this PR would also be beneficial to reduce the total size of the blocks by not appending empty blooms at all.

This PR also fixes the maximum block size since we did not look at the unflushed index and blooms pages size.

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

[chaudum]

nit

Suggested change

	series[i].Blooms = v2.NewSliceIter([]*Bloom{})
	series[i].Blooms = v2.NewEmptyIter[*Bloom]()