lib/promscrape/discovery/consul: allow stale responses from Consul service discovery by default

This aligns with Prometheus behaviour.

See `allow_stale` option description at https://prometheus.io/docs/prometheus/latest/configuration/configuration/#consul_sd_config

docs/CHANGELOG.md

@@ -23,6 +23,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * FEATURE: improve performance for registering new time series in `indexdb` by up to 50%. Thanks to @ahfuzhang for [the issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2249).
 * FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent.html): add service discovery for [Yandex Cloud](https://cloud.yandex.com/en/). See [these docs](https://docs.victoriametrics.com/sd_configs.html#yandexcloud_sd_configs) and [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1386).
 
+* BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): allow stale responses from Consul service discovery (aka [consul_sd_configs](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#consul_sd_config)) by default in the same way as Prometheus does. This should reduce load on Consul when discovering big number of targets. Stale responses can be disabled by specifying `allow_stale: false` option in `consul_sd_config`.
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): set `up` metric to `0` for partial scrapes in [stream parsing mode](https://docs.victoriametrics.com/vmagent.html#stream-parsing-mode). Previously the `up` metric was set to `1` when at least a single metric has been scraped before the error. This aligns the behaviour of `vmselect` with Prometheus.
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): restart all the scrape jobs during [config reload](https://docs.victoriametrics.com/vmagent.html#configuration-update) after `global` section is changed inside `-promscrape.config`. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2884).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): properly assume role with AWS ECS credentials. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2875). Thanks to @transacid for [the fix](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/2876).

lib/promscrape/discovery/consul/consul.go

@@ -27,7 +27,7 @@ type SDConfig struct {
 	Tags              []string                   `yaml:"tags,omitempty"`
 	NodeMeta          map[string]string          `yaml:"node_meta,omitempty"`
 	TagSeparator      *string                    `yaml:"tag_separator,omitempty"`
-	AllowStale        bool                       `yaml:"allow_stale,omitempty"`
+	AllowStale        *bool                      `yaml:"allow_stale,omitempty"`
 	// RefreshInterval time.Duration `yaml:"refresh_interval"`
 	// refresh_interval is obtained from `-promscrape.consulSDCheckInterval` command-line option.
 }

lib/promscrape/discovery/consul/watch.go

@@ -45,7 +45,7 @@ type serviceWatcher struct {
 // newConsulWatcher creates new watcher and starts background service discovery for Consul.
 func newConsulWatcher(client *discoveryutils.Client, sdc *SDConfig, datacenter, namespace string) *consulWatcher {
 	baseQueryArgs := "?dc=" + url.QueryEscape(datacenter)
-	if sdc.AllowStale {
+	if sdc.AllowStale == nil || *sdc.AllowStale {
 		baseQueryArgs += "&stale"
 	}
 	if namespace != "" {