This should reduce CPU load during scraping when target discovery generates
big number of `__meta_*` labels (for instance, k8s discovery).
See https://www.robustperception.io/life-of-a-label for details.
Prometheus docs at https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config say:
> In communication with external systems, they are always applied only
> when a time series does not have a given label yet and are ignored otherwise.
Though this may result in consistency chaos when scrape targets override `external_labels`,
let's stick with Prometheus behavior for the sake of backwards compatibility.
There is last resort in vmagent with `-remoteWrite.label`, which consistently
sets the configured labels to all the metrics before sending them to remote storage.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/366
