Previously byte slices up to 2^20 bytes (e.g. 1Mb) were cached because of a typo in the commit c14dafce43 .
This could result in increased memory usage when vmagent scrapes many regular targets, which expose
relatively small number of metrics (e.g. up to a few thousand per target) and a few large targets such as kube-state-metrics,
which expose more than 10 thousand metrics. This is common case for Kubernetes monitoring.
While at it, remove pools for very small byte slices, since they are rarely used during scraping.
Do not cache too big byte buffers and too big writeRequestCtx objects,
since it is cheaper to re-create them instead of wasting RAM for their caching.
This reverts 7f6f350ee1
Previously targets with big number of metrics and/or labels could generated too big buffers,
which then could be re-used when scraping targets with small number of metrics.
This resulted in memory waste.
Now big buffers are used only for targets with big number of metrics / labels,
while small buffers are used for targets with small number of metrics / labels.
