Without reset, labels duplicates could have been added during stream aggregation.
Since `ctx.Labels` is reused during processing of many series, each series will
add its labels to the context. Even if the same labels were already addeded on prev
iteration. Now, we reset `ctx.Labels` on each iteration to contain so labels from
different series didn't interfere.
This could have cause exceeding of the limit on number of labels per pushed time series.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4277
Signed-off-by: hagen1778 <roman@victoriametrics.com>