Handle histogram's created timestamp

Signed-off-by: Arthur Silva Sens <arthursens2005@gmail.com>

storage/remote/write_handler.go

@@ -397,7 +397,8 @@ func (h *writeHandler) appendV2(app storage.Appender, req *writev2.Request, rs *
 		allSamplesSoFar := rs.AllSamples()
 		var ref storage.SeriesRef
 
-		if h.ingestCTZeroSample {
+		// Samples.
+		if h.ingestCTZeroSample && len(ts.Samples) > 0 {
 			// CT only needs to be ingested for the first sample, it will be considered
 			// out of order for the rest.
 			ref, err = h.handleCTZeroSample(app, ref, ls, ts.Samples[0], ts.CreatedTimestamp, rs)
@@ -405,8 +406,6 @@ func (h *writeHandler) appendV2(app storage.Appender, req *writev2.Request, rs *
 				h.logger.Debug("Error when appending CT in remote write request", "err", err, "series", ls.String(), "created_timestamp", ts.CreatedTimestamp, "timestamp", ts.Samples[0].Timestamp)
 			}
 		}
-
-		// Samples.
 		for _, s := range ts.Samples {
 			ref, err = app.Append(ref, ls, s.GetTimestamp(), s.GetValue())
 			if err == nil {
@@ -427,6 +426,14 @@ func (h *writeHandler) appendV2(app storage.Appender, req *writev2.Request, rs *
 		}
 
 		// Native Histograms.
+		if h.ingestCTZeroSample && len(ts.Histograms) > 0 {
+			// CT only needs to be ingested for the first histogram, it will be considered
+			// out of order for the rest.
+			ref, err = h.handleHistogramZeroSample(app, ref, ls, ts.Histograms[0], ts.CreatedTimestamp, rs)
+			if err != nil {
+				h.logger.Debug("Error when appending CT in remote write request", "err", err, "series", ls.String(), "created_timestamp", ts.CreatedTimestamp, "timestamp", ts.Histograms[0].Timestamp)
+			}
+		}
 		for _, hp := range ts.Histograms {
 			if hp.IsFloatHistogram() {
 				ref, err = app.AppendHistogram(ref, ls, hp.Timestamp, nil, hp.ToFloatHistogram())
@@ -511,6 +518,29 @@ func (h *writeHandler) handleCTZeroSample(app storage.Appender, ref storage.Seri
 	return ref, err
 }
 
+// handleHistogramZeroSample appends CT as a zero-value sample with CT value as the sample timestamp.
+// It doens't return errors in case of out of order CT.
+func (h *writeHandler) handleHistogramZeroSample(app storage.Appender, ref storage.SeriesRef, l labels.Labels, hist writev2.Histogram, ct int64, rs *WriteResponseStats) (storage.SeriesRef, error) {
+	var err error
+	if hist.Timestamp != 0 && ct != 0 {
+		if hist.IsFloatHistogram() {
+			ref, err = app.AppendHistogramCTZeroSample(ref, l, hist.Timestamp, ct, nil, hist.ToFloatHistogram())
+		} else {
+			ref, err = app.AppendHistogramCTZeroSample(ref, l, hist.Timestamp, ct, hist.ToIntHistogram(), nil)
+		}
+		if err == nil {
+			rs.Histograms++
+		}
+		if err != nil && errors.Is(err, storage.ErrOutOfOrderCT) {
+			// Even for the first sample OOO is a common scenario because
+			// we can't tell if a CT was already ingested in a previous request.
+			// We ignore the error.
+			err = nil
+		}
+	}
+	return ref, err
+}
+
 // NewOTLPWriteHandler creates a http.Handler that accepts OTLP write requests and
 // writes them to the provided appendable.
 func NewOTLPWriteHandler(logger *slog.Logger, appendable storage.Appendable, configFunc func() config.Config) http.Handler {