This eliminates the need in .(*T) casting for results obtained from Load()
Leave atomic.Value for map, since atomic.Pointer[map[...]...] makes double pointer to map,
because map is already a pointer type.
The number of series per target can be limited with the following options:
* Global limit with `-promscrape.maxSeriesPerTarget` command-line option.
* Per-target limit with `max_series: N` option in `scrape_config` section.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1561
These numbers are exposed via the following metrics:
- vmagent_hourly_series_limit_current_series
- vmagent_daily_series_limit_current_series
Expose also the limits via the following metrics:
- vmagent_hourly_series_limit_max_series
- vmagent_daily_series_limit_max_series
