lib/promscrape: show only unhealthy targets if show_only_unhealthy filter is enabled (#6960)

### Describe Your Changes

It is better to show only unhealthy targets instead of all of them when
`show_only_unhealthy` filter is enabled.
Related issue:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3536

### Checklist

The following checks are **mandatory**:

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Co-authored-by: Roman Khavronenko <roman@victoriametrics.com>

docs/changelog/CHANGELOG.md

@@ -28,6 +28,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE: [vmui](https://docs.victoriametrics.com/#vmui): change request method for `/query_range` and `/query` calls from `GET` to `POST`. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6288).
 * FEATURE: [dashboards](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/dashboards) for VM single-node, cluster, vmalert, vmagent, VictoriaLogs: add `Go scheduling latency` panel to show the 99th quantile of Go goroutines scheduling. This panel should help identifying insufficient CPU resources for the service. It is especially useful if CPU gets throttled, which now should be visible on this panel.
 * FEATURE: [alerts](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/deployment/docker/alerts-health.yml): add alerting rule to track the Go scheduling latency for goroutines. It should notify users if VM component doesn't have enough CPU to run or gets throttled.
+* FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/) and [Single-node VictoriaMetrics](https://docs.victoriametrics.com/): hide jobs that contain only healthy targets when `show_only_unhealthy` filter is enabled. Before, jobs without unhealthy targets were still displayed on the page. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3536).
 
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent/) fix service discovery of Azure Virtual Machines for response contains `nextLink` in `Host:Port` format. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6912).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent/): properly consume messages [from kafka](https://docs.victoriametrics.com/vmagent/#kafka-integration). Previously vmagent could skip some messages during start-up.

lib/promscrape/targetstatus.go

@@ -475,6 +475,9 @@ func (tsm *targetStatusMap) getTargetsStatusByJob(filter *requestFilter) *target
 			}
 			targetsStatuses = append(targetsStatuses, ts)
 		}
+		if filter.showOnlyUnhealthy && len(targetsStatuses) == 0 {
+			continue
+		}
 		jts = append(jts, &jobTargetsStatuses{
 			jobName:       jobName,
 			upCount:       ups,
@@ -488,7 +491,7 @@ func (tsm *targetStatusMap) getTargetsStatusByJob(filter *requestFilter) *target
 	emptyJobs := getEmptyJobs(jts, jobNames)
 	var err error
 	jts, err = filterTargets(jts, filter.endpointSearch, filter.labelSearch)
-	if len(filter.endpointSearch) > 0 || len(filter.labelSearch) > 0 {
+	if len(filter.endpointSearch) > 0 || len(filter.labelSearch) > 0 || filter.showOnlyUnhealthy {
 		// Do not show empty jobs if target filters are set.
 		emptyJobs = nil
 	}