[cmd/telemetrygen] expose the generated span duration as a command pa… (

open-telemetry#29116)

**Description:** 

As originally proposed in open-telemetry#26991 before I got distracted

Exposes the duration of generated spans as a command line parameter. It
uses a `DurationVar` flag so units can be easily provided and are
automatically applied.

Example usage:

```bash
telemetrygen traces --traces 100 --otlp-insecure --span-duration 10ns # nanoseconds
telemetrygen traces --traces 100 --otlp-insecure --span-duration 10us # microseconds
telemetrygen traces --traces 100 --otlp-insecure --span-duration 10ms # milliseconds
telemetrygen traces --traces 100 --otlp-insecure --span-duration 10s # seconds
```

**Testing:** 

Ran without the argument provided `telemetrygen traces --traces 1
--otlp-insecure` and seen spans publishing with the default value.

Ran again with the argument provided: `telemetrygen traces --traces 1
--otlp-insecure --span-duration 1s`

And observed the expected output:

```
Resource SchemaURL: https://opentelemetry.io/schemas/1.4.0
Resource attributes:
     -> service.name: Str(telemetrygen)
ScopeSpans #0
ScopeSpans SchemaURL: 
InstrumentationScope telemetrygen 
Span #0
    Trace ID       : 8b441587ffa5820688b87a6b511d634c
    Parent ID      : 39faad428638791b
    ID             : 88f0886894bd4ee2
    Name           : okey-dokey
    Kind           : Server
    Start time     : 2023-11-12 02:05:07.97443 +0000 UTC
    End time       : 2023-11-12 02:05:08.97443 +0000 UTC
    Status code    : Unset
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-client)
Span #1
    Trace ID       : 8b441587ffa5820688b87a6b511d634c
    Parent ID      : 
    ID             : 39faad428638791b
    Name           : lets-go
    Kind           : Client
    Start time     : 2023-11-12 02:05:07.97443 +0000 UTC
    End time       : 2023-11-12 02:05:08.97443 +0000 UTC
    Status code    : Unset
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-server)
	{"kind": "exporter", "data_type": "traces", "name": "debug"}
```

**Documentation:** No documentation added.

---------

Co-authored-by: Pablo Baeyens <pbaeyens31+github@gmail.com>

.chloggen/feat-fake-span-durations.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: telemetrygen
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Exposes the span duration as a command line argument `--span-duration`
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [29116]
+
+# (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:
+
+# 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: [user]

cmd/telemetrygen/internal/traces/config.go

@@ -4,6 +4,8 @@
 package traces
 
 import (
+	"time"
+
 	"github.com/spf13/pflag"
 
 	"github.com/open-telemetry/opentelemetry-collector-contrib/cmd/telemetrygen/internal/common"
@@ -18,6 +20,7 @@ type Config struct {
 	StatusCode       string
 	Batch            bool
 	LoadSize         int
+	SpanDuration     time.Duration
 }
 
 // Flags registers config flags.
@@ -32,4 +35,5 @@ func (c *Config) Flags(fs *pflag.FlagSet) {
 	fs.StringVar(&c.StatusCode, "status-code", "0", "Status code to use for the spans, one of (Unset, Error, Ok) or the equivalent integer (0,1,2)")
 	fs.BoolVar(&c.Batch, "batch", true, "Whether to batch traces")
 	fs.IntVar(&c.LoadSize, "size", 0, "Desired minimum size in MB of string data for each trace generated. This can be used to test traces with large payloads, i.e. when testing the OTLP receiver endpoint max receive size.")
+	fs.DurationVar(&c.SpanDuration, "span-duration", 123*time.Microsecond, "The duration of each generated span.")
 }

cmd/telemetrygen/internal/traces/traces.go

@@ -132,6 +132,7 @@ func Run(c *Config, logger *zap.Logger) error {
 			wg:               &wg,
 			logger:           logger.With(zap.Int("worker", i)),
 			loadSize:         c.LoadSize,
+			spanDuration:     c.SpanDuration,
 		}
 
 		go w.simulateTraces(telemetryAttributes)

cmd/telemetrygen/internal/traces/worker.go

@@ -28,15 +28,14 @@ type worker struct {
 	totalDuration    time.Duration   // how long to run the test for (overrides `numTraces`)
 	limitPerSecond   rate.Limit      // how many spans per second to generate
 	wg               *sync.WaitGroup // notify when done
+	loadSize         int             // desired minimum size in MB of string data for each generated trace
+	spanDuration     time.Duration   // duration of generated spans
 	logger           *zap.Logger
-	loadSize         int
 }
 
 const (
 	fakeIP string = "1.2.3.4"
 
-	fakeSpanDuration = 123 * time.Microsecond
-
 	charactersPerMB = 1024 * 1024 // One character takes up one byte of space, so this number comes from the number of bytes in a megabyte
 )
 
@@ -46,11 +45,15 @@ func (w worker) simulateTraces(telemetryAttributes []attribute.KeyValue) {
 	var i int
 
 	for w.running.Load() {
+		spanStart := time.Now()
+		spanEnd := spanStart.Add(w.spanDuration)
+
 		ctx, sp := tracer.Start(context.Background(), "lets-go", trace.WithAttributes(
 			semconv.NetPeerIPKey.String(fakeIP),
 			semconv.PeerServiceKey.String("telemetrygen-server"),
 		),
 			trace.WithSpanKind(trace.SpanKindClient),
+			trace.WithTimestamp(spanStart),
 		)
 		sp.SetAttributes(telemetryAttributes...)
 		for j := 0; j < w.loadSize; j++ {
@@ -72,18 +75,19 @@ func (w worker) simulateTraces(telemetryAttributes []attribute.KeyValue) {
 			semconv.PeerServiceKey.String("telemetrygen-client"),
 		),
 			trace.WithSpanKind(trace.SpanKindServer),
+			trace.WithTimestamp(spanStart),
 		)
 		child.SetAttributes(telemetryAttributes...)
 
 		if err := limiter.Wait(context.Background()); err != nil {
 			w.logger.Fatal("limiter waited failed, retry", zap.Error(err))
 		}
 
-		opt := trace.WithTimestamp(time.Now().Add(fakeSpanDuration))
+		endTimestamp := trace.WithTimestamp(spanEnd)
 		child.SetStatus(w.statusCode, "")
-		child.End(opt)
+		child.End(endTimestamp)
 		sp.SetStatus(w.statusCode, "")
-		sp.End(opt)
+		sp.End(endTimestamp)
 
 		i++
 		if w.numTraces != 0 {

cmd/telemetrygen/internal/traces/worker_test.go

@@ -80,6 +80,38 @@ func TestRateOfSpans(t *testing.T) {
 	assert.True(t, len(syncer.spans) <= 20, "there should have been less than 20 spans, had %d", len(syncer.spans))
 }
 
+func TestSpanDuration(t *testing.T) {
+	// prepare
+	syncer := &mockSyncer{}
+
+	tracerProvider := sdktrace.NewTracerProvider()
+	sp := sdktrace.NewSimpleSpanProcessor(syncer)
+	tracerProvider.RegisterSpanProcessor(sp)
+	otel.SetTracerProvider(tracerProvider)
+
+	targetDuration := 1 * time.Second
+	cfg := &Config{
+		Config: common.Config{
+			Rate:          10,
+			TotalDuration: time.Second / 2,
+			WorkerCount:   1,
+		},
+		SpanDuration: targetDuration,
+	}
+
+	// sanity check
+	require.Len(t, syncer.spans, 0)
+
+	// test
+	require.NoError(t, Run(cfg, zap.NewNop()))
+
+	for _, span := range syncer.spans {
+		startTime, endTime := span.StartTime(), span.EndTime()
+		spanDuration := endTime.Sub(startTime)
+		assert.Equal(t, targetDuration, spanDuration)
+	}
+}
+
 func TestUnthrottled(t *testing.T) {
 	// prepare
 	syncer := &mockSyncer{}