Fix fetched chunk from store size in metric

[dannykopping]

What this PR does / why we need it:
The Size() function on chunk.Chunk calls out to func (ChunkRef) Size() int in the proto, which doesn't actually include the size of the chunk data! We need to call chunk.Data.Size() for that.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
This will fix the Cache.Chunk.BytesSent stat which indicates how many chunk bytes we sent to the chunk cache. This should probably be renamed for clarity at some point; it's a little confusing.

The same fix is applies to the loki_chunk_fetcher_fetched_size_bytes{source="store"} histogram metric.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[MichelHollands]

LGTM. Nice catch!

[grafanabot]

Hello @dannykopping!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[grafanabot]

Hello @dannykopping!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[grafanabot]

The backport to release-2.7.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-8971-to-release-2.7.x origin/release-2.7.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x de84737368914fa240108c0a3ef843eb5b5e4d3b
# Push it to GitHub
git push --set-upstream origin backport-8971-to-release-2.7.x
git switch main
# Remove the local backport branch
git branch -D backport-8971-to-release-2.7.x

Then, create a pull request where the base branch is release-2.7.x and the compare/head branch is backport-8971-to-release-2.7.x.