frontend.cache-split-interval is unnecessary and adds misconfiguration pitfall.

[owen-d]

What

The frontend.cache-split-interval is a ripe target for reducing configuration complexity in cortex. It's currently used to generate cache keys and is downstream of the SplitBy middleware.

I believe it should instead inherit from the SplitBy configuration in order to guarantee they're aligned. Having two configurations that should be equivalent only seems to leave room for configuration error.

Implementation path

Since this is a current configuration, we can mark it deprecated and when not specified defer to the SplitBy parameter.