Refine the exported "chainDensity" metrics to use different timeframes

[nfrisby]

Another request from @karknu. So that SPOs can see their node struggling sooner rather than later.

The current logic:

• Updates the metric here https://github.com/IntersectMBO/cardano-node/blob/8db5cb3147e577667d8fefbe31ee93e00307c801/cardano-node/src/Cardano/Tracing/Tracers.hs#L1026

• Calculates the number here https://github.com/IntersectMBO/cardano-node/blob/8db5cb3147e577667d8fefbe31ee93e00307c801/cardano-node/src/Cardano/Node/Tracing/Tracers/ChainDB.hs#L2758

The current calculation looks to be "how long it took to grow by 2160 blocks".

Instead, people might want to monitor "how many blocks grew in the last X minutes" for various values of X (10 min? 30 min? 1 hr? 3 hr? etc)