Cache sanitized label names and return them next time.
This reduces the number of allocations and speeds up the SanitizeLabelName()
function for common case when the number of unique label names is smaller than 100k
* fixes service discovery for kubernetes
now it must take in account all pods that belong to the discovered endpoint and endpointslice
adds simple test for endpoints
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2134
* wip
* docs/CHANGELOG.md: document the change
Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>
