store-gateway: Extend block load logs to include basic block info

[alexweav]

What this PR does

This is a tiny change that just adds extra info to the log when a block is loaded during the sync process.
It can answer questions like "how often are we loading L2 blocks," at the point where the blocked is synced instead of needing to wait for a query to touch the block.

It also includes extra general info about the block, so we can get an idea of size and not need to cross reference the ULID against other logs or the bucket.

Which issue(s) this PR fixes or relates to

Supporting https://github.com/grafana/mimir-squad/issues/3373

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

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Note

Low Risk
Log-only change that adds extra fields on successful block load; no functional, security, or data-path behavior changes.

Overview
Extends the store-gateway addBlock() success log line to include basic block metadata when a block is loaded during sync.

The loaded new block log now emits compaction_level, num_series, out-of-order flag (ooo), and the block time range (from/to), improving observability without changing load behavior.

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[narqo]

One minor comment, but I agree in spirit 🔥

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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