Document how to calculate the minimum kafka retention / replay offset

[replay]

There are a lot of factors which need to be considered when calculating the minimum offset to replay on startup. We should document them somewhere, because it's really easy to forget about one.

• Time window for restarts due to maintenance
• Max chunk span
• Chunk stale time
• GC Interval
• Reorder buffer duration

Did I forget one?

[Dieterbe]

the two places that I know of where we describe this are:

• https://github.com/grafana/metrictank/blob/master/docs/operations.md#data-doesnt-show-up
• chunk-max-stale / metrics-max-stale per retention #614

I agree. a dedicated place in the docs to explain this, would be nice.

[stale]

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