### Describe Your Changes
doing similar changes for both vmagent and vminsert (like one in
https://github.com/VictoriaMetrics/VictoriaMetrics/pull/7399) ends up
with almost same implementations for each of packages instead of having
this shared code in one place. one of the reasons is the same Timeseries
and Labels structure from different prompb and prompbmarshal packages.
My proposal is to use structures from prompb package only to
marshal/unmarshal sent/received data, but for internal transformations
use only structures from prompbmarshal package
Another example, where it already can help to simplify code is streaming
aggregation pipeline for vmsingle (now it first marshals
prompb.Timeseries to storage.MetricRow and then if streaming aggregation
or deduplication is enabled it unmarshals all the series back but to
prompbmarshal.Timeseries)
### Checklist
The following checks are **mandatory**:
- [ ] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
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