lib/promscrape: respect 0 value for series_limit param (#5663)

* lib/promscrape: respect `0` value for `series_limit` param

Respect `0` value for `series_limit` param in `scrape_config`
even if global limit was set via `-promscrape.seriesLimitPerTarget`.
Previously, `0` value will be ignored in favor of `-promscrape.seriesLimitPerTarget`.

This behavior aligns with possibility to override `series_limit` value via
relabeling with `__series_limit__` label.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

* Update docs/CHANGELOG.md

---------

Signed-off-by: hagen1778 <roman@victoriametrics.com>
Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>

docs/CHANGELOG.md

@@ -76,6 +76,7 @@ The sandbox cluster installation is running under the constant load generated by
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): properly assume role with [AWS IRSA authorization](https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html). Previously role chaining was not supported. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3822) for details.
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): exit if there is config syntax error in [`scrape_config_files`](https://docs.victoriametrics.com/vmagent.html#loading-scrape-configs-from-multiple-files) when `-promscrape.config.strictParse=true`. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5508).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): properly discover targets for `role: endpoints` and `role: endpointslice` in [kubernetes_sd_configs](https://docs.victoriametrics.com/sd_configs.html#kubernetes_sd_configs). Previously some `endpoints` and `endpointslice` targets could be left undiscovered or some targets could have missing `__meta_*` labels when performing service discovery in busy Kubernetes clusters with large number of pods. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5557).
+* BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): respect explicitly set `series_limit: 0` in [scrape_config](https://docs.victoriametrics.com/sd_configs.html#scrape_configs). This allows removing [`series_limit` restriction](https://docs.victoriametrics.com/vmagent.html#cardinality-limiter) on a per-`scrape_config` basis when global limit is set via `-promscrape.seriesLimitPerTarget`. Previously, `0` value was ignored in favor of `-promscrape.seriesLimitPerTarget`.
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix a link for the statistic inaccuracy explanation in the cardinality explorer tool. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5460).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix the display of autocomplete results and cache the results. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5472) and [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5470).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): send `step` param for instant queries. The change reverts [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3896) due to reasons explained in [this comment](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3896#issuecomment-1896704401).

lib/promscrape/config.go

@@ -322,7 +322,7 @@ type ScrapeConfig struct {
 	StreamParse         bool                       `yaml:"stream_parse,omitempty"`
 	ScrapeAlignInterval *promutils.Duration        `yaml:"scrape_align_interval,omitempty"`
 	ScrapeOffset        *promutils.Duration        `yaml:"scrape_offset,omitempty"`
-	SeriesLimit         int                        `yaml:"series_limit,omitempty"`
+	SeriesLimit         *int                       `yaml:"series_limit,omitempty"`
 	NoStaleMarkers      *bool                      `yaml:"no_stale_markers,omitempty"`
 	ProxyClientConfig   promauth.ProxyClientConfig `yaml:",inline"`
 
@@ -876,8 +876,8 @@ func getScrapeWorkConfig(sc *ScrapeConfig, baseDir string, globalCfg *GlobalConf
 		noStaleTracking = *sc.NoStaleMarkers
 	}
 	seriesLimit := *seriesLimitPerTarget
-	if sc.SeriesLimit > 0 {
-		seriesLimit = sc.SeriesLimit
+	if sc.SeriesLimit != nil {
+		seriesLimit = *sc.SeriesLimit
 	}
 	swc := &scrapeWorkConfig{
 		scrapeInterval:       scrapeInterval,

lib/promscrape/config_test.go

@@ -1190,6 +1190,29 @@ scrape_configs:
 			jobNameOriginal: "foo",
 		},
 	})
+
+	defaultSeriesLimitPerTarget := *seriesLimitPerTarget
+	*seriesLimitPerTarget = 1e3
+	f(`
+scrape_configs:
+- job_name: foo
+  series_limit: 0
+  static_configs:
+  - targets: ["foo.bar:1234"]
+`, []*ScrapeWork{
+		{
+			ScrapeURL:       "http://foo.bar:1234/metrics",
+			ScrapeInterval:  defaultScrapeInterval,
+			ScrapeTimeout:   defaultScrapeTimeout,
+			jobNameOriginal: "foo",
+			Labels: promutils.NewLabelsFromMap(map[string]string{
+				"instance": "foo.bar:1234",
+				"job":      "foo",
+			}),
+			SeriesLimit: 0,
+		},
+	})
+	*seriesLimitPerTarget = defaultSeriesLimitPerTarget
 }
 
 func equalStaticConfigForScrapeWorks(a, b []*ScrapeWork) bool {