lib/promscrape: make a copy of ScrapeWork from discovered []ScrapeWork slice instead of referring to an item in this slice

This should prevent from holding previously discovered []ScrapeWork slices when a part of discovered targets changes over time.
This should reduce memory usage for the case when big number of discovered scrape targets changes over time.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/825

docs/CHANGELOG.md

@@ -2,6 +2,8 @@
 
 # tip
 
+* FEATURE: vmagent: reduce memory usage when service discovery detects big number of scrape targets and the set of discovered targets changes over time.
+  See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/825
 * FEATURE: vmagent: add `-promscrape.dropOriginalLabels` command-line option, which can be used for reducing memory usage when scraping big number of targets.
   See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/825#issuecomment-724308361 for details.
 * FEATURE: vmalert: explicitly set extra labels to alert entities. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/870

lib/promscrape/targetstatus.go

@@ -64,7 +64,7 @@ func (tsm *targetStatusMap) Reset() {
 func (tsm *targetStatusMap) Register(sw *ScrapeWork) {
 	tsm.mu.Lock()
 	tsm.m[sw.ID] = targetStatus{
-		sw: sw,
+		sw: *sw,
 	}
 	tsm.mu.Unlock()
 }
@@ -78,7 +78,7 @@ func (tsm *targetStatusMap) Unregister(sw *ScrapeWork) {
 func (tsm *targetStatusMap) Update(sw *ScrapeWork, group string, up bool, scrapeTime, scrapeDuration int64, err error) {
 	tsm.mu.Lock()
 	tsm.m[sw.ID] = targetStatus{
-		sw:             sw,
+		sw:             *sw,
 		up:             up,
 		scrapeGroup:    group,
 		scrapeTime:     scrapeTime,
@@ -221,7 +221,7 @@ type jobStatus struct {
 }
 
 type targetStatus struct {
-	sw             *ScrapeWork
+	sw             ScrapeWork
 	up             bool
 	scrapeGroup    string
 	scrapeTime     int64