cmd: Make feature flag `native-histograms` a no-op.

Signed-off-by: beorn7 <beorn@grafana.com>

cmd/prometheus/main.go

@@ -255,11 +255,7 @@ func (c *flagConfig) setFeatureListOptions(logger *slog.Logger) error {
 				parser.ExperimentalDurationExpr = true
 				logger.Info("Experimental duration expression parsing enabled.")
 			case "native-histograms":
-				// Change relevant global variables. Hacky, but it's hard to pass a new option or default to unmarshallers.
-				t := true
-				config.DefaultConfig.GlobalConfig.ScrapeNativeHistograms = &t
-				config.DefaultGlobalConfig.ScrapeNativeHistograms = &t
-				logger.Warn("This option for --enable-feature is being phased out. It currently changes the default for the scrape_native_histograms scrape config setting to true, but will become a no-op in v3.9+. Stop using this option and set scrape_native_histograms in the scrape config instead.", "option", o)
+				logger.Warn("This option for --enable-feature is a no-op. To scrape native histograms, set the scrape_native_histograms scrape config setting to true.", "option", o)
 			case "ooo-native-histograms":
 				logger.Warn("This option for --enable-feature is now permanently enabled and therefore a no-op.", "option", o)
 			case "created-timestamp-zero-ingestion":
@@ -564,7 +560,7 @@ func main() {
 	a.Flag("scrape.discovery-reload-interval", "Interval used by scrape manager to throttle target groups updates.").
 		Hidden().Default("5s").SetValue(&cfg.scrape.DiscoveryReloadInterval)
 
-	a.Flag("enable-feature", "Comma separated feature names to enable. Valid options: exemplar-storage, expand-external-labels, memory-snapshot-on-shutdown, promql-per-step-stats, promql-experimental-functions, extra-scrape-metrics, auto-gomaxprocs, native-histograms, created-timestamp-zero-ingestion, concurrent-rule-eval, delayed-compaction, old-ui, otlp-deltatocumulative, promql-duration-expr, use-uncached-io, promql-extended-range-selectors. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details.").
+	a.Flag("enable-feature", "Comma separated feature names to enable. Valid options: exemplar-storage, expand-external-labels, memory-snapshot-on-shutdown, promql-per-step-stats, promql-experimental-functions, extra-scrape-metrics, auto-gomaxprocs, created-timestamp-zero-ingestion, concurrent-rule-eval, delayed-compaction, old-ui, otlp-deltatocumulative, promql-duration-expr, use-uncached-io, promql-extended-range-selectors. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details.").
 		Default("").StringsVar(&cfg.featureList)
 
 	a.Flag("agent", "Run Prometheus in 'Agent mode'.").BoolVar(&agentMode)

docs/command-line/prometheus.md

@@ -58,7 +58,7 @@ The Prometheus monitoring server
 | <code class="text-nowrap">--query.timeout</code> | Maximum time a query may take before being aborted. Use with server mode only. | `2m` |
 | <code class="text-nowrap">--query.max-concurrency</code> | Maximum number of queries executed concurrently. Use with server mode only. | `20` |
 | <code class="text-nowrap">--query.max-samples</code> | Maximum number of samples a single query can load into memory. Note that queries will fail if they try to load more samples than this into memory, so this also limits the number of samples a query can return. Use with server mode only. | `50000000` |
-| <code class="text-nowrap">--enable-feature</code> <code class="text-nowrap">...<code class="text-nowrap"> | Comma separated feature names to enable. Valid options: exemplar-storage, expand-external-labels, memory-snapshot-on-shutdown, promql-per-step-stats, promql-experimental-functions, extra-scrape-metrics, auto-gomaxprocs, native-histograms, created-timestamp-zero-ingestion, concurrent-rule-eval, delayed-compaction, old-ui, otlp-deltatocumulative, promql-duration-expr, use-uncached-io, promql-extended-range-selectors. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details. |  |
+| <code class="text-nowrap">--enable-feature</code> <code class="text-nowrap">...<code class="text-nowrap"> | Comma separated feature names to enable. Valid options: exemplar-storage, expand-external-labels, memory-snapshot-on-shutdown, promql-per-step-stats, promql-experimental-functions, extra-scrape-metrics, auto-gomaxprocs, created-timestamp-zero-ingestion, concurrent-rule-eval, delayed-compaction, old-ui, otlp-deltatocumulative, promql-duration-expr, use-uncached-io, promql-extended-range-selectors. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details. |  |
 | <code class="text-nowrap">--agent</code> | Run Prometheus in 'Agent mode'. |  |
 | <code class="text-nowrap">--log.level</code> | Only log messages with the given severity or above. One of: [debug, info, warn, error] | `info` |
 | <code class="text-nowrap">--log.format</code> | Output format of log messages. One of: [logfmt, json] | `logfmt` |

docs/feature_flags.md

@@ -45,20 +45,6 @@ statistics. Currently this is limited to totalQueryableSamples.
 When disabled in either the engine or the query, per-step statistics are not
 computed at all.
 
-## Native Histograms
-
-`--enable-feature=native-histograms`
-
-_This feature flag is being phased out. You should not use it anymore._
-
-Native histograms are a stable feature by now. However, to scrape native
-histograms, a scrape config setting `scrape_native_histograms` is required. To
-ease the transition, this feature flag sets the default value of
-`scrape_native_histograms` to `true`. From v3.9 on, this feature flag will be a
-true no-op, and the default value of `scrape_native_histograms` will be always
-`false`. If you are still using this feature flag while running v3.8, update
-your scrape configs and stop using the feature flag before upgrading to v3.9.
-
 ## Experimental PromQL functions
 
 `--enable-feature=promql-experimental-functions`

docs/migration.md

@@ -43,18 +43,19 @@ This document offers guidance on migrating from Prometheus 2.x to Prometheus 3.0
   Prometheus v3 will log a warning if you continue to pass these to
   `--enable-feature`.
 
-- Starting from Prometheus version v3.8, the feature flag `native-histograms` is
-  deprecated. Use the new `scrape_native_histograms` global and per-scrape
-  configuration option instead.
+- Starting from v3.9, the feature flag `native-histograms` is a no-op. Native
+  histograms are a stable feature now, but scraping them has to be enabled via
+  the `scrape_native_histograms` global or per-scrape configuration option
+  (added in v3.8).
 
 ## Configuration
 
-- The scrape job level configuration option `scrape_classic_histograms` has been
-  renamed to `always_scrape_classic_histograms`. If you use the
-  `--enable-feature=native-histograms` feature flag to ingest native histograms
-  and you also want to ingest classic histograms that an endpoint might expose
-  along with native histograms, be sure to add this configuration or change your
-  configuration from the old name.
+- The scrape job level configuration option `scrape_classic_histograms` has
+  been renamed to `always_scrape_classic_histograms`. If you use the
+  `scrape_native_histograms` scrape configuration option to ingest native
+  histograms and you also want to ingest classic histograms that an endpoint
+  might expose along with native histograms, be sure to add this configuration
+  or change your configuration from the old name.
 - The `http_config.enable_http2` in `remote_write` items default has been
   changed to `false`. In Prometheus v2 the remote write http client would
   default to use http2. In order to parallelize multiple remote write queues