fix(helm): add flush=true to preStop hook

[alextricity25]

What this PR does / why we need it:
WAL needs to be flushed per https://github.com/grafana/loki/blob/main/docs/sources/operations/storage/wal.md#how-to-scale-updown

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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[jkroepke]

Hi @alextricity25

Thanks for your PR. Could you please:

• rebase your PR
• add notes to the changelog
• use fix(helm) as prefix

[jkroepke]

LGTM

[a-abella]

According to the Loki docs, /ingester/shutdown defaults flush to true. https://grafana.com/docs/loki/latest/reference/loki-http-api/#flush-in-memory-chunks-and-shut-down

URL query parameters:
flush=: Flag to control whether to flush any in-memory chunks the ingester holds. Defaults to true.

I define my own /ingester/shutdown lifecycle prestop hook on my single-AZ distrbuted mode ingesters, so I'm curious why this change was needed and if I would need it also.

[jkroepke]

I followed the documentation https://github.com/grafana/loki/blob/main/docs/sources/operations/storage/wal.md#how-to-scale-updown that explicit mentions the endpoint with flush=true, but it did mention that the parameter is enabled by default.

So your right, technically is not necessary, but it clarifies the situation. Feel free to create a PR for the single-AZ distrbuted mode ingesters.