tracer: Log a warning on NoopTracer.StartSpan() (#2991)

Author: mtoffl01

Diff for: contrib/internal/httptrace/httptrace_test.go

@@ -360,6 +360,8 @@ func BenchmarkStartRequestSpan(b *testing.B) {
 		b.Errorf("Failed to create request: %v", err)
 		return
 	}
+	// Avoid (NoopTracer) StartSpan warning log
+	log.UseLogger(log.DiscardLogger{})
 	opts := []ddtrace.StartSpanOption{
 		tracer.ServiceName("SomeService"),
 		tracer.ResourceName("SomeResource"),

Diff for: ddtrace/internal/globaltracer.go

@@ -9,6 +9,7 @@ import (
 	"sync/atomic"
 
 	"gopkg.in/DataDog/dd-trace-go.v1/ddtrace"
+	"gopkg.in/DataDog/dd-trace-go.v1/internal/log"
 )
 
 var (
@@ -46,6 +47,7 @@ type NoopTracer struct{}
 
 // StartSpan implements ddtrace.Tracer.
 func (NoopTracer) StartSpan(_ string, _ ...ddtrace.StartSpanOption) ddtrace.Span {
+	log.Warn("Tracer must be started before starting a span; Review the docs for more information: https://docs.datadoghq.com/tracing/trace_collection/library_config/go/")
 	return NoopSpan{}
 }
 

Diff for: ddtrace/tracer/tracer_test.go

@@ -6,13 +6,15 @@
 package tracer
 
 import (
+	"bytes"
 	"context"
 	"encoding/base64"
 	"encoding/binary"
 	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
+	llog "log"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -2696,3 +2698,29 @@ func TestExecutionTraceSpanTagged(t *testing.T) {
 	assert.Equal(t, partialSpan.Meta["go_execution_traced"], "partial")
 	assert.NotContains(t, untracedSpan.Meta, "go_execution_traced")
 }
+
+func TestNoopTracerStartSpan(t *testing.T) {
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("Failed to create pipe: %v", err)
+	}
+
+	undo := log.UseLogger(customLogger{l: llog.New(w, "", llog.LstdFlags)})
+	defer undo()
+
+	StartSpan("abcd")
+
+	w.Close()
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+
+	log := buf.String()
+	expected := "Tracer must be started before starting a span"
+	assert.Contains(t, log, expected)
+}
+
+type customLogger struct{ l *llog.Logger }
+
+func (c customLogger) Log(msg string) {
+	c.l.Print(msg)
+}