Add IsPartial to Querystats for partial-response header

[manute]

Describe Your Changes

#718
Add IsPartial to Querystats for partial-response header

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• My change adheres to VictoriaMetrics contributing guidelines.
• My change adheres to VictoriaMetrics development goals.

[func25]

Thanks for the PR, this is a good start, but the current approach is mixing two different concepts:

– allow_partial_response is a user option that controls how we handle errors from some storage nodes.
– IsPartial (and the VL-Partial-Response header) should reflect the actual outcome of the query: whether the response was built from all backends or some were unavailable and we returned a partial result.

In other words, IsPartial shouldn't just mirror allow_partial_response. It needs to be derived from the cluster error‑handling logic (where we already know which vlstorage nodes failed and whether we still returned a result) and then aggregated across nodes via QueryStats. Once that is in place, the HTTP handlers can read QueryStats.IsPartial and set VL-Partial-Response appropriately.