prometheusreceiver: deprecate start time adjustment (#37879)

#### Description

Start time metric adjustment is still enabled by default, but can now be
disabled using a feature gate.

It is being replaced by the metricstarttime processor:
#37186

#### Link to tracking issue

Part of
#37186

#### Testing

I updated the unit test for counters

#### Documentation

Updated the README

Author: dashpole

.chloggen/prometheus-deprecate-adjuster.yaml

@@ -0,0 +1,28 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: deprecation
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: prometheusreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Deprecate metric start time adjustment in the prometheus receiver. It is being replaced by the metricstarttime processor.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37186]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: Start time adjustment is still enabled by default. To disable it, enable the |
+  receiver.prometheusreceiver.RemoveStartTimeAdjustment feature gate.
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/prometheusreceiver/README.md

@@ -90,6 +90,12 @@ prometheus --config.file=prom.yaml
 "--feature-gates=receiver.prometheusreceiver.RemoveLegacyResourceAttributes"
 ```
 
+- `receiver.prometheusreceiver.RemoveStartTimeAdjustment`: If enabled, the prometheus receiver no longer sets the start timestamp of metrics if it is not known. Use the `metricstarttime` processor instead if you need this functionality.
+
+```shell
+"--feature-gates=receiver.prometheusreceiver.RemoveStartTimeAdjustment"
+```
+
 - `report_extra_scrape_metrics`: Extra Prometheus scrape metrics can be reported by setting this parameter to `true`
 
 You can copy and paste that same configuration under:

receiver/prometheusreceiver/internal/metricfamily.go

@@ -145,7 +145,7 @@ func (mg *metricGroup) toDistributionPoint(dest pmetric.HistogramDataPointSlice)
 	tsNanos := timestampFromMs(mg.ts)
 	if mg.created != 0 {
 		point.SetStartTimestamp(timestampFromFloat64(mg.created))
-	} else {
+	} else if !removeStartTimeAdjustment.IsEnabled() {
 		// metrics_adjuster adjusts the startTimestamp to the initial scrape timestamp
 		point.SetStartTimestamp(tsNanos)
 	}
@@ -223,7 +223,7 @@ func (mg *metricGroup) toExponentialHistogramDataPoints(dest pmetric.Exponential
 	tsNanos := timestampFromMs(mg.ts)
 	if mg.created != 0 {
 		point.SetStartTimestamp(timestampFromFloat64(mg.created))
-	} else {
+	} else if !removeStartTimeAdjustment.IsEnabled() {
 		// metrics_adjuster adjusts the startTimestamp to the initial scrape timestamp
 		point.SetStartTimestamp(tsNanos)
 	}
@@ -317,7 +317,7 @@ func (mg *metricGroup) toSummaryPoint(dest pmetric.SummaryDataPointSlice) {
 	point.SetTimestamp(tsNanos)
 	if mg.created != 0 {
 		point.SetStartTimestamp(timestampFromFloat64(mg.created))
-	} else {
+	} else if !removeStartTimeAdjustment.IsEnabled() {
 		// metrics_adjuster adjusts the startTimestamp to the initial scrape timestamp
 		point.SetStartTimestamp(tsNanos)
 	}
@@ -331,7 +331,7 @@ func (mg *metricGroup) toNumberDataPoint(dest pmetric.NumberDataPointSlice) {
 	if mg.mtype == pmetric.MetricTypeSum {
 		if mg.created != 0 {
 			point.SetStartTimestamp(timestampFromFloat64(mg.created))
-		} else {
+		} else if !removeStartTimeAdjustment.IsEnabled() {
 			// metrics_adjuster adjusts the startTimestamp to the initial scrape timestamp
 			point.SetStartTimestamp(tsNanos)
 		}

receiver/prometheusreceiver/internal/metrics_adjuster.go

@@ -260,6 +260,9 @@ func NewInitialPointAdjuster(logger *zap.Logger, gcInterval time.Duration, useCr
 // AdjustMetrics takes a sequence of metrics and adjust their start times based on the initial and
 // previous points in the timeseriesMap.
 func (a *initialPointAdjuster) AdjustMetrics(metrics pmetric.Metrics) error {
+	if removeStartTimeAdjustment.IsEnabled() {
+		return nil
+	}
 	for i := 0; i < metrics.ResourceMetrics().Len(); i++ {
 		rm := metrics.ResourceMetrics().At(i)
 		_, found := rm.Resource().Attributes().Get(semconv.AttributeServiceName)

receiver/prometheusreceiver/internal/starttimemetricadjuster.go

@@ -60,6 +60,9 @@ func NewStartTimeMetricAdjuster(logger *zap.Logger, startTimeMetricRegex *regexp
 }
 
 func (stma *startTimeMetricAdjuster) AdjustMetrics(metrics pmetric.Metrics) error {
+	if removeStartTimeAdjustment.IsEnabled() {
+		return nil
+	}
 	startTime, err := stma.getStartTime(metrics)
 	if err != nil {
 		if !useCollectorStartTimeFallbackGate.IsEnabled() {

receiver/prometheusreceiver/internal/transaction.go

@@ -19,6 +19,7 @@ import (
 	"github.com/prometheus/prometheus/storage"
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/consumer"
+	"go.opentelemetry.io/collector/featuregate"
 	"go.opentelemetry.io/collector/pdata/pcommon"
 	"go.opentelemetry.io/collector/pdata/pmetric"
 	"go.opentelemetry.io/collector/receiver"
@@ -29,6 +30,13 @@ import (
 	mdata "github.com/open-telemetry/opentelemetry-collector-contrib/receiver/prometheusreceiver/internal/metadata"
 )
 
+var removeStartTimeAdjustment = featuregate.GlobalRegistry().MustRegister(
+	"receiver.prometheusreceiver.RemoveStartTimeAdjustment",
+	featuregate.StageAlpha,
+	featuregate.WithRegisterDescription("When enabled, the Prometheus receiver will"+
+		" leave the start time unset. Use the new metricstarttime processor instead."),
+)
+
 type resourceKey struct {
 	job      string
 	instance string
@@ -465,9 +473,11 @@ func (t *transaction) Commit() error {
 		return nil
 	}
 
-	if err = t.metricAdjuster.AdjustMetrics(md); err != nil {
-		t.obsrecv.EndMetricsOp(ctx, dataformat, numPoints, err)
-		return err
+	if !removeStartTimeAdjustment.IsEnabled() {
+		if err = t.metricAdjuster.AdjustMetrics(md); err != nil {
+			t.obsrecv.EndMetricsOp(ctx, dataformat, numPoints, err)
+			return err
+		}
 	}
 
 	err = t.sink.ConsumeMetrics(ctx, md)