otelhttp: Allow setting start time using context (#6137)

Fixes
#5318

This allows overriding the start time for http server metrics and
traces. See the linked issue for the motivation.

---------

Co-authored-by: Robert Pająk <[email protected]>

CHANGELOG.md

@@ -13,6 +13,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Transform raw (`slog.KindAny`) attribute values to matching `log.Value` types.
   For example, `[]string{"foo", "bar"}` attribute value is now transformed to `log.SliceValue(log.StringValue("foo"), log.StringValue("bar"))` instead of `log.String("[foo bar"])`. (#6254)
 - Add the `WithSource` option to the `go.opentelemetry.io/contrib/bridges/otelslog` log bridge to set the `code.*` attributes in the log record that includes the source location where the record was emitted. (#6253)
+- Add `ContextWithStartTime` and `StartTimeFromContext` to `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp`, which allows setting the start time using go context. (#6137)
 
 ### Fixed
 

instrumentation/net/http/otelhttp/handler.go

@@ -117,6 +117,11 @@ func (h *middleware) serveHTTP(w http.ResponseWriter, r *http.Request, next http
 		}
 	}
 
+	if startTime := StartTimeFromContext(ctx); !startTime.IsZero() {
+		opts = append(opts, trace.WithTimestamp(startTime))
+		requestStartTime = startTime
+	}
+
 	ctx, span := tracer.Start(ctx, h.spanNameFormatter(h.operation, r), opts...)
 	defer span.End()
 

instrumentation/net/http/otelhttp/start_time_context.go

@@ -0,0 +1,29 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package otelhttp // import "go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
+
+import (
+	"context"
+	"time"
+)
+
+type startTimeContextKeyType int
+
+const startTimeContextKey startTimeContextKeyType = 0
+
+// ContextWithStartTime returns a new context with the provided start time. The
+// start time will be used for metrics and traces emitted by the
+// instrumentation. Only one labeller can be injected into the context.
+// Injecting it multiple times will override the previous calls.
+func ContextWithStartTime(parent context.Context, start time.Time) context.Context {
+	return context.WithValue(parent, startTimeContextKey, start)
+}
+
+// StartTimeFromContext retrieves a time.Time from the provided context if one
+// is available. If no start time was found in the provided context, a new,
+// zero start time is returned and the second return value is false.
+func StartTimeFromContext(ctx context.Context) time.Time {
+	t, _ := ctx.Value(startTimeContextKey).(time.Time)
+	return t
+}

instrumentation/net/http/otelhttp/start_time_context_test.go

@@ -0,0 +1,23 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package otelhttp
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStartTimeFromContext(t *testing.T) {
+	ctx := context.Background()
+	startTime := StartTimeFromContext(ctx)
+	assert.True(t, startTime.IsZero())
+
+	now := time.Now()
+	ctx = ContextWithStartTime(ctx, now)
+	startTime = StartTimeFromContext(ctx)
+	assert.True(t, startTime.Equal(now))
+}

instrumentation/net/http/otelhttp/test/handler_test.go

@@ -12,6 +12,7 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -72,6 +73,9 @@ func assertScopeMetrics(t *testing.T, sm metricdata.ScopeMetrics, attrs attribut
 		},
 	}
 	metricdatatest.AssertEqual(t, want, sm.Metrics[2], metricdatatest.IgnoreTimestamp(), metricdatatest.IgnoreValue())
+
+	// verify that the custom start time, which is 10 minutes in the past, is respected.
+	assert.GreaterOrEqual(t, sm.Metrics[2].Data.(metricdata.Histogram[float64]).DataPoints[0].Sum, float64(10*time.Minute/time.Millisecond))
 }
 
 func TestHandlerBasics(t *testing.T) {
@@ -102,6 +106,9 @@ func TestHandlerBasics(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	// set a custom start time 10 minutes in the past.
+	startTime := time.Now().Add(-10 * time.Minute)
+	r = r.WithContext(otelhttp.ContextWithStartTime(r.Context(), startTime))
 	h.ServeHTTP(rr, r)
 
 	rm := metricdata.ResourceMetrics{}
@@ -138,6 +145,7 @@ func TestHandlerBasics(t *testing.T) {
 	if got, expected := string(d), "hello world"; got != expected {
 		t.Fatalf("got %q, expected %q", got, expected)
 	}
+	assert.Equal(t, startTime, spans[0].StartTime())
 }
 
 func TestHandlerEmittedAttributes(t *testing.T) {