[serve.llm][dashboard] Configure aggregation interval

[eicherseiji]

Why are these changes needed?

• Make aggregation interval configurable
• Fix TPOT/TTFT units

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[alanwguo]

Code itself looks good,

my feedback is more of a UX question which you guys are the owners of so approving to unblock.

[alanwguo]

will users actually know what this means and why they should customize it?

Alternatively, you can use the grafana built in variables $__rate_interval or $__interval which is set by grafana based on what timerange they select.

I think in the grafana UI, you can explicitly override the intervals as well and it will apply it to all the promql if you use the $__interval variable.